I doubt either the GTX 260 or 4870 can run on a OEM 400W PSU.
Please RTFM before posting crap next time, this is even worse than that Jacknoryar dude copying & pasting the best graphics cards for the money without changing the $80 from 9600GT to 9800GT.
Before buying any PSU, make sure you have room in your case. Some proprietary PSU's are non-standard lengths. I once bought a replacement PSU for a friend's PC, only to discover it was 2 inches to large to fit in his case.