Will a GTX 260 work with my psu?

These are the PSU requirements for that GPU :

Minimum of a 500 Watt power supply.
(Minimum recommended power supply with +12 Volt current rating of 36 Amp Amps.)
Two available 6-pin Molex hard drive power dongles

so the answer is yes it will work but don't expect it to power anything else since you are pushing it =)

You might be pushing it with a superclock, since you gotta account for the CPU and extra stuff your running. When your stressing your card watch it cause the card is going to use a lot of power. Plus, the peak for a norm 500w PSU is 500w not that it consistently performs at that power level.