Why do you need more RAM? You haven't given us any clues to the problems that make you think you need more. Here are a few common possibilities.
System generally slow?
Windows doesn't use most of the memory your computer has, it writes stuff out to a swap file. This can grow huge, hundreds of Mb, as it keeps every temporary thing in there forever. Performance can be greatly improved by killing the swap file to force Win to make a new empty one.
Go to Control Panel, System, look for Virtual Memory settings (probably under an Advanced tab, sorry I can't check exactly where right now as I'm not on Windows). Uncheck the box "Let Windows manage the memory(recommended)". Then check "manage the memory myself" and put in about 50Mb for minimum and 200Mb for maximum. Apply and restart the computer. You will then find a file Win386.swp file in c:\
You will notice this stays the same size: it can grow while being used to the size you specified but is cleared out when you shut down. So it always only has today's stuff in it making memory swapping a lot faster.
Out of Memory or Insufficient Memory messages?
This can happen, usually on older computers, if you're trying to do a HUGE memory-intensive operation, such as trying to calculate 200,000 cells of inter-dependent formulae on a large spreadsheet. Or copying many large graphics via the Clipboard. The problem is not physical memory but how much the program concerned can handle internally. Solutions: break the spreadsheet job into smaller sections, clear the clipboard before adding more stuff to it. Usually this has warning symptoms, like very slow (sometimes even patchy) redrawing of the screen when you change pages or a dialog box or menu is closed.
Slow screen redraws? esp with graphics?
IF you use a graphics program (Corel, Photoshop etc) set it up NOT to render the whole screen while you're working. In some drawing programs you can't turn this off - stop using them. If the slowness is in games or videos, you need more VIDEO memory. The onboard card will only use the 128 it's designed for, so get a stronger add-on card(see below). IF the slowness is in video on the internet, the problem is in download speeds, not your memory.(Check this out by saving the video then viewing it directly from your own copy).
OnBoard video?
This means the video card is built in on your motherboard. It uses some (you say 128Mb) of your 512Mb leaving less for other programs to use. Video cards use memory to prepare the next screen, so it can be sent to your monitor in one lump, quickly. This is how you can watch TV or videos on your computer. Old ("old" = 1995) machines had HALF or ONE Mb video card memory and you could see the screen being drawn up line by line. IF you were a gamer you would want more than 128Mb but this should be enough for ordinary uses.
Added a video card?
So why did you add a video card? Maybe that card has less than 128Mb and this makes screen changes slow, which you blame on general memory? if you DID put in a separate card make sure the onboard one is DISABLED or the 128Mb will still be reserved for it. To disable it when the machine starts up press F1 or Esc or Ctrl+S (it will tell you at the bottom of the screen) to get into the bios setup, where you can hunt around to disable the onboard video. (DO NOT CHANGE ANYTHING ELSE even if you think you know what you're doing or your machine might not work again).
If you did NOT put in an add-on card, this might be a good option, if you put one that is stronger than 128Mb. It will cost more than adding to the general RAM but that won't solve any screen slowness problems; a better video card will.
Other onboard stuff?
your SOUND card might also be an onboard one, also stealing some of your 512 for its own use.
Generally slow performance?
This is much more likely junk, insufficient hard disk space and assorted viruses than not enough memory. Is the hard drive always running for a long time, with a scratching around sound, before anything happens?
Do a full system virus check. Also do a full system check for SPYWARE, which can slow you down no end by not letting you do what YOU want to do until they've dialed into their home planets to pass on info about what you're doing.
If your hard drive has less than 15% empty space it has to search around for empty slots to write to, any time you save anything. Delete any old stuff you never use anymore. If you're not sure, download a freeware duplicate checker, which will identify and ditch anything you have multiple copies of. When deleting hold down the SHIFT key so the files are truly deleted - otherwise they're just moved to the Recycle bin, which takes up the same amount of space. Reboot the computer to see any speed improvements from a big delete session; rebooting causes it to start looking for empty hard drive space from the beginning on the drive.
Defrag or not?
Defragmentation moves files around so they are on a continguous part of the disk; otherwise one file can be spread all over, in little pieces, using space freed up by earlier deleting of other files. Finding these slows down the computer when opening or closing files. Rightlick on your hard disk icon in My Computer and Properties Tools will get you to a defragment button.(or get in from "disk cleanup"). WARNING it will start defragging with no further notice. This is a dangerous and slow operation and worth doing only if you think your hard drive will be big enough for the next year or so. IF not then consider buying an extra hard drive (the new USB ones are easy to install and come in huge sizes). Once it is attached, COPY most of your directories over to it - everything except Windows. Do it in small batches. Then delete them all in your old drive. Reboot the machine and copy the files back. This takes a bit of time but you can do it with drag and drop. This way all the files end up in contiguous blocks on the disk. Your program files will run faster if all their bits are together, but once installed they won't grow. The first copying process pieces together the files for you; the second writes them back to the original disk in one piece. Your own files and directories on the other hand will grow as you add to them. Ideally put all your data (letters, downloaded stuff, graphics etc) onto a separate partition, so you can later defrag that partition only if you need to, without risking killing your programs.
If after all this you STILL think you need more RAM, it doesn't matter if it's one stick or more. With more than one, they will only run at the speed of the slowest one, IF the slowest one is in the first position on the board. If there is a faster one in the first slot it will run at the fast speed AS LONG AS that is all the memory you are using. As soon as you need to use the second (slower) stick it hits a brick wall: it might slow down or it might just crash your system. Cheapest is to get one other stick the same as the one you have now.
Next new machine you get, don't get with onboard video and audio. It will cost a bit more to get separate cards but you get better choice and better upgradeability.
Good luck