So the system works but it is just unstable. Then with the card removed the system only reboots (and is thus non-functional)... did you remove the card while the system was on or sumthin :S?
Sounds bad
. The first bit sounded very much like the PSU was the culprit, but the second bit is rather weird... sounds like a settings problem in windows or something is defective. Honestly if the system works without vid card but crashes random with it (could be the card itself is defective) it really seems like the PSU simply isn't pulling it.