If you're into modifications and not particularly concern about how pretty it looks. It might be cheaper to just get another 600W to power 2 cards. The overall efficiency would also save you on running costs. Otherwise, I'd suggest you just put in a bit more dollar and go for the 1KW which puts the peak load at around 62~65% (PSU last longer and will be more energy efficient) and gives you headroom to add a 4th card or overclock more heavily if you need to.