nope. you cannot SLI a GTX 670 and a GTX 590.
But the GTX 590 VERY close to a GTX 670's performance... (a small bit faster) so.. you can probably use either one as a physix card.
and a 750W PSU is definitely enough for both cards. (as long as it has the necessary plugs)
You'll need 2x 8 pins and 2x 6 pins to run both the GTX 670 and GTX 590.
Okay, thanks, I think I'll stick with my 670 so I could get another one later.