When it comes to memory, a computer can’t have too much.  These days, I recommend, at the very least, 1GB of memory to run Windows XP or Vista.  I recommend 2GB for regular use, and more than that if you are doing intensive stuff.   Memory is currently quite inexpensive, and if you only have 256MB or 512MB you will definitely see a performance boost by upgrading.

One thing to keep in mind, and where people get confused, is when they upgrade their computer to 4GB memory, but the computer reports only 3.5GB.  Most computers have a 32-bit edition of Windows, regardless of what the hardware is capable of.  32-bit operating systems are unable to read over 4GB memory TOTAL — which includes any “invisible” cache memory, etc… thus only a portion of your new memory is available.  64-bit operating systems can see much more, so you shouldn’t have a problem… however there are other issues with 64-bit such as driver & application incompatibilities that you may have to check out.

If you would like to attempt to upgrade memory on your own, it’s fairly simple, but you need to ensure you get the right kind.  All forms of DDR memory are interchangable (ie. PC3200 memory will work in a system requesting PC2700)… but are not compatible with DDR2.  DDR & DDR2 are completely different, and have a different pinout so you can’t accidentally put the wrong chip in a slot.

With that said, here is a nice video which shows how to upgrade your memory:

httpv://www.youtube.com/watch?v=SiFIgSQOY7g