since his motherboard have only 1 Pci Express slot, and since you have got an AMD Athlon 7750 X2 CPU and 4GB DDR2 RAM, any card up to a GTX 650 will be a waste... So, in my opinion a GTX 650 would be perfect for his rig. SLI/Crossfire with this harware is impossible, and I suggest you to get a single fast GPU than 2 mid range GPUs in SLI/Crossfire.
But I agree with Dwarf, GTX 650 doesn't support SLI (and this is very strange, since all mid-range GeForce GPUs supported SLI in the past, like GTX 550 Ti, GTS 450, GTS 250...) and this is really a bad thing... But since the HD 7770 has only 1GB RAM, I really suggest you to go for Nvidia.
I thik in your system, a single GTX 650 will be the better choice since you can't use more multi GPU technology. And, probably a GTX 650 will be a waste, because of your CPU.