hi mate
your packard bell imedia pc supports ddr memory and will only accept speeds of 333mhz /400mhz so this means you need to be looking for either pc3200(recommended(faster memory)) or pc2700 which is still ok but a bit slower than the pc3200
you can mix the memory and it should be fine( i have done this) however with some motherboard(its hard to predict which ones) you may experience crashes when using some apps and games
as for some compatible memory ,well i allways buy my memory from aria.co.uk ( http://www.aria.co.uk/) as they are the cheapest in the uk(online sales)
you pc has 4 dimms supporting up to 4gb of memory so my advice is fill up the remaining dimms with as much as you can,so for example if you currently have just 1gb of memory using up 2 of the 4 slots then buy 2 x 1gb ddr pc3200 modules to fill the remaining slots giving you a total of 3gb memory
this is ideal since 32bit systems will only recognize 3.5gb of memory and going with 3gb will ensure all the memory will be seen
so what memory should you buy ?
well theres the "AriaNet 1GB PC3200 DDR SDRAM " ( http://www.aria.co.uk/SuperSpecials/Other+products/AriaNet+1GB+PC3200+DDR+SDRAM+?productId=25701) and at just £21.79 inc. VAT this represents good value
however you may prefer to get a kit like the Corsair 2GB PC3200 Value Select (2x1GB) ( http://www.aria.co.uk/Products/Components/Memory/DDR/DDR400+(PC3200)/Corsair+2GB+PC3200+Value+Select+(2x1GB)+?productId=22287) however whichever way you buy it remember you only want up to 3gb so if you have just 512mb at the moment this may require the 2 x 1gb modules and a additional 512mb module aswell
as for installing them,they simply insert into the spare slots and make sure the clips at each end lock into place,however also when installing the memory allways put the larger of the memory in dimm 1 and dimm 2 and then the smaller capacity memory in the final dimm 3 and dimm 4 slots
your memory upgrade will make a massive improvement in performance especially when multi-tasking and gaming,and this will be more apparent if you are upgrading from just 512mb,however it will be less apparent if upgrading from 2gb to 3gb
for you reference and to assist you when purchasing your new memory here are the memory specifications for the packard bell i-media 1801/b :
# Four 184-pin DDR SDRAM DIMM sockets.
# Serial Presence Detect
# Support for single-sided or double-sided DIMMs (DDR 333 / DDR400)
# Support for up to 4 GB system memory
# Non-ECC RAM (ECC memory will run in non-ECC mode)
# Single and dual channel mode operation:
* Single channel mode with single DIMM, non-symmetrical population or non-identical DIMMs
* Dual channel mode with 2 or 4 identical DIMMs, populated symmetricall
and thats it,i hope this has helped however any problems or if you require anymore advice let me know
good luck mate !