I am not a hardcore PC gamer at the moment... mainly because my PC (as detailed in my profile) is not fast enough from a 9yr old's PC now.
What I can link you to, however, is
TigerDirect.com and
NewEgg.com. These are the two main Internet sources that I use to buy all my PC parts.
I highly recommend TigerDirect.com. The review's on that site are usually very good and detailed.
Do a search for your two cards and see if you can't find out anything.
Also, know this. The limiting factor on PC games now a days, IS NOT the GPU (the video card). It is the CPU! The GPU's have gotten so fast, that the CPU's cannot keep up, and therefore become the bottleneck. You can do an experiment. Open up your favorite PC game and the MS Taskmanager (Ctrl + Alt + Delete --> taskmanager). See if your CPU is maxed out when you're playing a game. If it is, upgrading the card may give you a little bit of extra frame rate/ less lag, but it may not help out as much as a new M/Bo and CPU.
Just something to keep in mind when upgrading. I did my upgrade (as detailed below) and it cost me $500! For that money I could have gotten something a lot better, but this machine is really my first and I didn't want to send it to the graveyard quite yet.
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AMD X2 5000+ Black w/ XFX GeForce 8200 MoBo
4GB OCZ DDR2 RAM
3/4 of a TB of data storage between 2 HDD's (no RAID)