Need help with a sli set up.....

This is going to be my first SLI set up and am wondering on the video cards. My system is as follows, asus p5n32-e sli plus board, q6600 processor, 3 gb kingston hyper x 6400 memory, 1 sata 36 gb operating system drive, 500 gb seagate, 250 sata w/d.Asus tv tuner card and front bay card reader with a antec 650 power supply. Now i figure i will need a bigger power supply so that i know. should i go for 2 320 gts or 640's????

Probably get the 2 640s.

Thank you, Do you think i should wait on the next gen card that is supposed to come out later this year. i have a 8600 gt for now tht plays great.