could i power another gtx 260 in sli with my power supply?
I believe you could do that. I think you'd be right at the top end of what that excellent PSU could do with your overclocks though. I'd recommend you back off the overclocking while doing the install and testing at stock before you push the CPU up again.
GTX 260 SLI review @ Tweaktown. Intel i7 920 @ 3.8GHz GIGABYTE EX58-UD5 2x Leadtek GTX 260 EXTREME+ factory OC in SLI. That 449watts at the wall socket works out to be about ~370watts DC from the PSU.