Ah, since you've already ordered 660Ti... We dont know what exact PSU you've got there. If it's not 80+ Bronze certified then you'd better think of buying a new one. Good 500W PSU would cost you about 60-70$.
660 Ti is difficult to find nowadays and btw this is not the card you should look for. You'd better go with a R9280X or GTX 770. If it's to expensive for you then you should look at R9 280 (without "X").
Regards