I'm just gonna copypasta random crud I found googling your problem:
The 6800 requires a power connector on the back of the card. Make sure you have that plugged and it is feeding enough power to the card. If it's also hooked up to other devices (hard drives, case fans, etc), try redoing the wiring in the case.
how long do you let it hang? sometimes I have found, if you jump the gun and restart it manually right away, you miss an informative BSOD that might take a bit to show up. With the family's pc here at home, I was testing oblivion, and it seemed to lock up, no response from anything. But then I got a phone call, and i forgot to reboot it, came back into the room, and there was a BSOD there telling me that the vidoe card got stuck in a loop, and updating the video drivers was the solution. you can also try updating the bios, running memtest, check for vga overheating.. unfortunately, there are a lot of reasons why this could be happening
It could be a ram problem, assuming you have 2 modules, remove one then test computer for lock ups.
If removing the first one stops the lock up then the second stick you just removed was bad, if it didnt stop the lock ups remove the other stick and put the first one back in, then test. If its still locking up its not a ram problem.
memtest86. Also, when you installed the new CPU did you reformat? I know it's a pain in the ass, but I always found it neccessary to do so.