I have a graphics card (Radeon 5700 or 5800 -- desktop pc version, I can't recall which number that is) which works mostly fine, except that when the cpu is under load and the graphics card fan is running it tends to restart the whole computer.
My bios post screen sometimes gives me messages after these resets about power surges.
I can mostly eliminate this behaviour by underclocking the graphics card to minimums and underclocking the cpu. (Phenom II)
The power supply is the problem here right? It's 800 watts but something must be wonky about it.
Or... is this the way graphics card problems sometimes symptomize themselves?
A common problem with 'generic' PSUs is that even the high wattage ones do not put out consistent voltages/amps when under increased load. They work fine under normal load. I would say it's definitely your PSU.
I would suggest purchasing a known brand like OCZ, Corsair, or CoolerMaster, that have a 80+ Bronze rating (higher, if you're using extremely expensive components). Any of their 600 W PSUs will provide better performance than a generic 800+ W PSU.
You're also running the risk of damaging your components.