Man you might wanna think about kicking up to a Seasonic 600w or at least a TP-II 550 with dualie 7900GTs and a (probably overclocked, yes?) Opty 144. Overclocking is gonna stress your PSU, and a coupla 7900GTs won't exactly be gentle either.
If you wanna do SLI AND overclock, the only PSU under 550W I'd recommend would be the PC Power and Cooling 510W model. You don't wanna be limited in your OC or lockup in games because your PSU can't feed the CPU and/or GPUs properly. I'd shoot for a minimum of 550w just to be safe.