What 750W PSU do you have? Most 500W units are fine for a single graphics card, so unless you got a super bad 750W unit you have more than enough power available.
A GTX 460 and a 5770 both need two slots next to each other in the case, but only need one PCI-E 16x slot. What case do you have? Looking at your motherboard, unless you have something in both PCI-E 1x slots you will have enough space.