No, your psu will be fine with 2 cards. I'm not sure if you can do 3 GTX 295 sli'd because they're already dual gpus by themselves.
But yeah, you can get another GTX 295 and SLI it with your other one. To be safe I'd say getting a slightly better PSU, maybe one with a little more amperage available. But 750W at 60A should do fine.
May of next year, I need this thing rocking awesomely! Anyway, the PhysX allows me to mix and match cards, which means I could put in a 300 series when they come out and it will work I guess?
I really probably won't, but I just want to have mroe stuff that is outstanding in a gaming PC. I have considered water cooling but it is not worth it..?