I'm recently considering the option of swapping my EVGA Geforce 8800GT 512mb (@ 650 mhz core and 1900 mhz memory*) to a HD4870. My budget is anything under $300 and my motherboard is a P35 (so no crossfire). The resolution that I'm going to be playing at is 1680*1050 on a 22" monitor.
I'm in a dilemma as to whether or not I should upgrade, I won $400 from Vegas and figured I might as well spend some money to enhance my rig, but then there hasn't been any comprehensive benchmark tests that I could see how big the gap is between a 8800GT and a HD4870 at 1680*1050 resolutions.
Maybe I should upgrade my CPU as well, but then it won't be the bottleneck factor for my games.
The HD4870 is two tiers above the 8800GT on Cleeve's hierarchy chart. He suggests that any less than a three-tier upgrade isn't that big a difference. You might improve your experience in front of your PC in other ways. Consider a nice new highback chair, and/or a new computer desk. Do you have a UPS? How about a nice-sized external drive for backups? Other peripherals?
500 Watt or greater power supply with two 75W 6-pin PCI Express® power connectors recommended (600 Watt and four 6-pin connectors for ATI CrossFireX™ technology in dual mode)
Certified power supplies are recommended. Refer to http://ati.amd.com/certifiedPSU for a list of Certified products.
For your current setup, I don't think that replacing your video card is a good idea. Depending upon the games and settings you use, at 1680x1050 you'd only see perhaps a 40-75% increase in performance; the higher end of that range is if you use AA in Crysis. Really, for that much money, I'd recommend just keeping your 8800GT for now; it certainly isn't showing much in the way of signs of age in games, now is it? I'm pretty certain the $300US would have a better-felt effect spent elsewhere, be it toward your computer or something else.