Hi, may i know which is the most suitable upgrade gpu for my pc? Here is my pc specs: Core 2 duo e7400, 2gb ddr2 ram, 320 hdd, normal 450w psu and screen resolution 1366 x 768. I'm currently using win xp and planning to upgrade to win 7. My current gpu is 9400 gt and thinking of upgrading my gpu to hd 6670 or gt 640. Any other recommendation for gpu that not going to bottleneck my cpu cos am not planning to upgrade anything else beside gpu or ram. Nvidia or ati gpu is better for light gaming? My budget for gpu is around $80.
What PSU do you have? You can probably run the 7770 just fine, but not necessarily. Especially with old models, the wattage can be deceptive in that it's sometimes much more focused on the +5 and +3.3V rails than it should be considering that modern CPUs and graphics cards almost purely use the +12V rails and many such units are not able to handle even a 7770 safely.
You can get a very nice Antec VP-450 PSU (it has a six pin PCIe connector; you don't need an eight pin for the 7770) for between $32 and $40 depending on if it's on sale or not.
If you can't afford a 7750, then the 6670 DDR3 is your next best option (The GDDR5 version is better, but too expensive), but it consumes only a little bit less power than the 7770 and much more than the 7750, so a new PSU might be important if yours is as bad as it seems.
Using the MIR, you should be able to afford the 6670 that bigcyco1 suggested (I checked and it seems to be the best graphics card that you can afford under these circumstances). This PSU is almost as good as the VP-450. However, if you post your PSU model, we can check for sure if it is adequate already. Maybe it is and if so, you won't need to spend money on a PSU and can instead get a Radeon 6670 GDDR5 or maybe something better.