Ok, I would like to upgrade my video card, as it its getting older. And I am starting to have some problems with it. I am really excited about the new call of duty coming out so it needs to be able to play that on relatively high settings. I have around 125 dollars to spend, give or take 20 or so. I would also like to NOT buy another psu to power this thing, I have a 430w Antec Earthwatts power supply. I am thinking I would like to try Nvidia, but not content on that.
Current System specs:
CPU - AMD athlon X2 7550 @ 3.1
GPU - HIS HD 4830 512mb
RAM - 4 gigs 1066 ocz ddr2
Radeon 4850/GTS 250, whichever you can find cheaper. Anything above those and you'll need to upgrade your PSU. ATI reccommends a 450w or greater for a 4850 but I think you'll be okay w/your 430w Antec. Nvidia also reccommends a 450w for a GTS 250. Either one would be a nice upgrade for you!
I'm pretty sure the 4870 would do just fine on an Antec 430 PSU with your CPU. You can find a few of those within your price range on Newegg and it's likely the best GPU your specs can take. But like Sparky suggested, the 4850 is a rock solid performer for ~$100.
I wouldn't really suggest the GTS 250 though. It does perform slightly better on average than the 4850...but the thing costs around that of a 4870. And the 4870 is a much better GPU.