Hey so First off how much did you spend on the system and is it too late to return it? J/K!!!
You do have an available slot on your motherboard but the 8800 wouldn't really fit and you do NEED another PSU...also upgrading to vista and getting a motherboard/cpu bundle from Tigerdirect or Newegg.com would be the ideal place to get something. I built my in-laws a "budget" computer for under $200 us, and I can play most ANY game out right now (crysis, and prototype, not so much...Low settings with SOME lage, but it plays and these are the games to benchmark!)
If you can find a college kid in your area that wants a little extra gas money, you could very easily get someone to build one for you. In your case you wouldn't really need to get a super expensive graphics card to play those games, but something like a 9600 gt 512 card would be fine for what you need and these go for about 40 dollars.
RAM is a different story, since your running XP it is out of date a bit, and I am guessing you have DDR ram, or being hopeful, DDR2, if that is the case you can get a couple sticks of DDR2 Ram 4gb total (only if you upgrade to Vista ), for a wopping $35 bucko's!
Also what is you processor and motherboard type? To find your CPU just go to "System" in the control panel and copy all that info. It should say your core, total ram, and processor speed. Tell me what all this is, and I'll let you know what you need to get and even how to install all of it, if your interested.
You can PM me all this info, and I'll look at it and get back to you.
BTW;
I am a Computer Engineer (Hardware Engineer) and have tons of experience building computers, so you have questions, I'll answer them...
-Ricardo