Athlon II X3 435 2.9GHz
2 x 2GB DDR3 1333mhz ram
Nvidia GTS 250 1GB green edition (Inno3d)
My brother has a 9600GT that batters Team fortress 2 on high all round 2xAA on the same resolution as me (1280x1024) or his might even be slightly higher. And he has no issues with his, mine however, supposedly the superior card, is struglling.
However its not just gaming, the PC feels sluggish all round, I have installed ALL drivers for the new build. I have also done virus scans and these dont pick anything up. Despite it affecting my PC all round I just get the feeling its something to do with the GPU, when my PC stops responding for a few seconds every now and then the screen always looks like its caught mid-fade,making me think its a graphics fault. I even OC'd my Athlon to 3.2GHz and its made no difference. When It was first built the gaming was fine, and it normally ( I say normally as it isnt always) is ok for about half hour. But then it goes very very laggy. Ive checked the temps also and even after gaming for an hour plus the GPU isnt above 55 and the CPU anything above 45.
run a good registry scan like cc and advance system care free edition. make sure case is clean inside no dust and make sure you got plenty of case fans and cables neat and well routed good luck. A good power supply doesn't hurt eighter. Gts 250 about as fast as a 9800gtx+ both decent cards but not rockets by all means
Try swapping cards with your brother and see what happens. They use the same drivers so it should be easy enough to do so as a test. If your systems still shows the same issues while he gets a performance boost from the better card then at least you've ruled out the card as the problem.