Stop getting GT's, they blow. Get something in the GTX range from nVidia (GTX265 and stronger), or 4800 Series from ATi. (4870 and stronger).
But you will have to get a new motherboard to support a new CPU and those GPU's fully, and a new power supply, but getting a good power supply means you won't have too later.
Here's a solid Corsiar 650 Watt, you won't really need that much power for running single card rigs, and this ones great because it has 52 amps on one rail.
http://www.newegg.com/Product/Produc...82E16817139005