As I said, if you are just using your computer for Sketchup modeling and not doing renders with some other GPU based programs it would not be necessary to upgrade your card(unless you have money to freely do so).
I have an 1100 watt power supply with 2 GTX 470's (2)6 pins per card. I think my next cards will be GTX770's I'll buy one and then another later. $300 a pop. smokin fast.