ATI Radeon HD 2600 Pro AGP BSOD and Other Errors

All right, here goes.

Around six months ago, I installed a new graphics card (Diamond Multimedia ATI Radeon HD 2600 Pro, AGP version) in my aging Dell Dimension 4600, along with another 2 GB of RAM. After struggling a bit with the graphics drivers, I managed to get it work. There were several blue screens, which continued for some time until they suddenly...stopped. Completely. Now, for some reason, they're back. Even the BSOD's are few and far between at this point (there were more this morning), and seem to be mostly solved by uninstalling the drivers and reinstalling them (for the sixth time). That problem has been replaced with programs "Encountering errors and needing to close", and general slowness. It is so slow, in fact, that when I attempt to open Steam to play some TF2, the "Store" tab remained completely white for a solid 20 minutes, until it just BSOD'd out.

Any help would be GREATLY appreciated.