Inside your computer is a large circuit board usually referred to as the "motherboard" or "mainboard." The RAM memory is attached to the motherboard, it just snaps into place in slots on the motherboard. Physically, the RAM memory is about the size of a tongue depressor stick, 3 to 4 inches long, 1/2" wide and 1/8" thick. It is typically called a "stick" of memory. You probably have one stick and your computer motherboard probably has 2 or 3 open slots for adding more sticks of memory.
The limits of RAM expansion is determined by the number of open slots and the capability of other components in your computer to access additional RAM. If your computer was manufactured in the last 5 years, it probably can accomodate expansion up to 2 GB of RAM.
Where to start. Your computer is a Hewlett Packard. Find the model number of the computer. Then go on line to HP.com and find information about your computer for that model and determine (1) how much memory can be added to this computer and (2) what type of RAM memory. You must add the same kind of memory as what is already in your computer. RAM memory has several characteristics you will want to match. If you can't find information on line, another easy way to determine your RAM is to open your computer (unplug the computer from the electrical outlet), find your RAM memory sticks, and take one out. Most motherboards have small snap release buttons at the ends of the slots (find a picture on Google). Read the model number on the side of the memory stick. Handle the memory by the edges. Try not to touch the gold or silver electrical contacts. Search Google with that number and you will find identical memory and description of the type, capacity and speed.
(a) Physical form and number of pins. RAM comes in several standard physical sizes and with various number of pins. Most desktop computers use RAM with either 184 pins or 240 pins (pins are the electrical contacts).
(b) type; your computer probably has either DDR or DDR2. There are other types available (SDRAM, RDRAM), but in the past couple years, DDR2 is the most common. All memory sticks must be the same type.
(c) Speed. Make sure all memory sticks are the same speed. Speed is described as PC2100, PC2700, PC3200, PC4200, PC5600, etc. It is also described as frequency of signal per second in Megahertz. For example, PC2100 is the same as 266MHz speed. Sometimes, you can upgrade your computer with faster speed memory (example, changing from PC2100 to PC2700), but there are always limits for each computer. It is not possible to upgrade from PC2100 to PC4200. The computer probably will not function, but if it does, you are wasting your money because the RAM will operate at a reduced speed, limited by other components. The user's manual for your computer should tell you the limits of RAM memory speed and capacity.
(d) ECN or NON-ECN. ECN stands for Error Correctiing Code and is found in servers and high-end computers. NON-ECN is common for most home use computers. You can use ENC memory in any computer, but if your computer requires ECN, you must use ECN memory.
(e) Capacity. You can add any size you want up to the limits of the computer access. Memory comes in capacity of 64MB (no longer manufactured), 128MB, 256MB, 512MB, 1GB, 2GB and 4GB.
If your computer can access up to 2GM, and you already have 1-stick of 256MB, then you can add 1 stick of 256MB and a second stick of 512MB to make a total of 1GB. Exception, if your computer has dual path memory access, then the existing memory is 2 sticks of 128MB and you must add memory in pairs of two identical sticks at a time. Add 2 more of 128MB and 2 of 256MB for a total of 1GB. Find the user's guide on HP.com for your computer.
Hope this helps.
Good luck