ehh, I'm sure he meant 400w or something.
Anyway, it's not all about wattage...if you have a crappy quality PSU and it has a weak +12v line then you're screwed.
If you've got an 800w power supply but your +12v line is only rated for 10a it still wouldn't run that card.
Your powersupply should have atleast 20-22a for a simple system with that card.
I've got a friend running a socket 754 board, a 3200+ CPU, a 6800GT AGP and his PSU is only 17a on the +12v which seems a bit low to me, but hell it runs it...still I'd recommend nothing less than 20a.
wouldn't be surprised if your PSU is 15a on the +12v line which is way too small.
you can find that info out by the sticker that's generally on the side of a PSU