I know this is somewhat a noob question, but do PCI - E 2.0 slots support ATI and NVidia cards, or just one or the other?
Also, which one is better for gaming (source games and games like Fallout 3 and CoD: Modern Warfare)? I really dont have much money either so I cant really get the high end cards. ) also BTW I had my eye on this http://www.newegg.com/Product/Product.aspx...N82E16883229114
But it got deactivated and now I'm looking again. That was pretty much my budget, so any suggestions on good computers for cheap? (sorry that that was off topic)
Also...
http://www.newegg.com/Store/SubCategory.as...erating-Systems
does the "for system builders" part effect this at all? Because I found this new PC with No OS and I was looking for an OS to install.... http://www.cyberpowerpc.com/system/Mega_Special_I/

All PCI-E 16X slots should support both ATI and NVidia cards. However, very few motherboards will support both SLI and crossfire.
I do not recommend that computer you showed us. The 9500GT is a terrible card for gaming.
This would be an excellent all-in-one solution.
Here is another good one.

PCI express is the type, any card that is designed to run PCI express x16 will run in any PCI express x16 slot, unless the board is configured SLI or crossfire, and even then, if your only using one card, an ATI card will go in a SLI board and vice versa, but if you plan on using dual cards, then you need to worry whether or not your board supports crossfire or SLI. and for gaming, I would recomend either a ATI 4870 (saphire) on up, or if your a diehard Nvidia fan (im not) at least the 260 (EVGA or PNY) but thats my advice. :D

Which computer do you not recommend?

Yes, those would be the perfect PC's (i wish I could afford one) but my budget is about 550$ WITH an OS. About 480 with out.

Can you build or do you need it pre-built?

Either of the above (listed by yourself) will work, but don't expect to be able to play on high quality.

Thanks, I will probably get the one with the GT 220 instead of the 9500 then.

Well, I have never built a computer, so I will probably stick with pre-built for now.