bunny, go to
www.pcpitstop.com and run the full battery of tests (you have to register - but its free and you won't get spammed -though you'll have to tell one page you don't want to download a trial version of PC Anywhere before it'll let you see your results). The Details page will tell you, among lots of other things, how much of what type of memory you have installed, and what the maximum amount of memory your motherboard/OS combo will support, how many memory sloys are occupied, and how many are available. You'll also get recommendations of things to do to improve the overall performance of your machine.
Both
Crucial and
Kingston , two big memory vendors, offer drill-down memory "configurators" that let you enter your machine vendor and model, then tell you what is the max your system supports, and what options they have to offer you. Since you've already got a half gig of RAM, I don't think its critical to add more; you'll get a bigger gameplay boost by upgrading your video card. Lots of 128MB AGP cards are available for around US$100, and there are tons of 64MB cards for way less. There are lots of vendors available. Check the review sites to get a feel for which ones offer which advantages and which drawbacks.
