i would suggest a 7800 GTX for your price range. its a very good card and will play all the latest games, but maybe not at the very highest setting. near highest though :]]
you power supply should be atleast 450 watt, no need to buy extra fans if you dont plan to overclock it (which i dont think you do), it takes one PCI-express slot.
[CPU: e6600 @ 3300 MHz] // [MB: ASUS P5N-E SLi] // [RAM: Patriot 'eased latency' PC2-6400 @ DDR800 5-4-4-12-2T] // [GPU: 8800GTX @ 650/1000/1600 (volt modded)] // [HDD: 2x Cavier 500GiB stripe] // [Chassis: Thermaltake Armor VA8000 (black)] // [PSU: Aspire(Apevia) 680w] // [OS: Windows 7 Ultimate x64][Cooling: water cooled GPU and CPU]
^ (9/7/2010): Ha, I remember when that was the stuff. My water cooling has since leaked and fried my mobo. So now I'm rocking an MSI laptop with an i5 430m :)