no you can't SLI them. to SLI 9600GT you need another 9600GT. the same goes with 9800GT. unlike ATI 4800 series you can CF any combination cards in 4800 series like pairing 4870 with 4830. the downside is 4870 performance in CF will be drag down by 4830. or you can do what mousemonkey said, use the 9800GT as the primary card and 9600GT for dedicated PhysX