Games freeze up in sli mode

I recently upgrade my computer from a single geforce 260 192 and i added an xfx geforce 260 216. It has worked fine for the last week but now i have been getting this random freeze up when close a game and launch another game. A few times i have received a page file out message from steam and I also get an error saying my graphics driver has crashed. All my games seem to be affected it doesn't matter which.

my system specs

q6600 overclock 3.1ghz

MSI P7N SLI Platinum LGA 775 NVIDIA nForce 750i SLI ATX Intel Motherboard

xfx geforce 260 216
bfg geforce 260 192

hard drive
2*500 gb western digital in raid 0

2*20x dvd burners

1 soundblaster sound card

silverstone 850watt power supply
power supply spec

+3.3V@25A,+5V@30A,+12V1@35A,+12V2@35A,
-12V@0.5A,+5VSB@3.0A

windows xp service pack 3
nvidia driver 180.08 whql

anything else please ask my gut feeling is that my power supply can not handle the system. I hope it isn't that i don't have money to replace the powersupply right now.

your two graphics cards aren't exactly the same,
so they don't play nice in SLI
you'll have to just use one card as a physics card and the other as the video card for it to run smoothly (unless NVIDIA comes out w/ a driver update that fixes that, but i doubt it)