Sli 7900GT OC + 8600GT

I have an Sli mother board with the 7900GT oc which has sli support, I was wondering about getting a second video card, and set up sli. Wouldit be okay if i combine my 7900GT with an 8600GT through sli? or that wont work? And can someone tell me which would be better conserning that 1 has 512mb and the other 256+ faster core/memory clock and a shader clock.

1. XFX GeForce 8600 GT Video Card - 512MB DDR3, PCI Express, SLI Ready, (Dual Link) Dual DVI, HDTV, Video Card PVT84JYDF3 in Canada at TigerDirect.ca

2. XFX GeForce 8600 GT XXX Video Card - 256MB DDR3, SLI Ready, PCI Express, (Dual Link) Dual DVI, HDTV, Video Card PVT84JUDD3 in Canada at TigerDirect.ca

PS. The 7900GT OC had a massive manufacture failure, all cards were sent back to recall to be fixed, when it returned, the cardcolour was changed from green to a blueish teal like colour. When I first bought it, the card had 256mb, but now I see alot of people with the same "7900 GT OC" saying it has 512mb.

Someone correct me if I am wrong, but the way it works is YES, you can SLI two different cards, but they have to have the same model GPU installed on the board. Being one is a 7 series and one an 8 series I doubt they do.
The other thing is the system will only run the two cards at the speed of the slowest card. You should see all the memory, but the highest core and memory clock will automatically be lowered to the same level as the lowest card.

For the best results, always use identical cards. Problems can arise for you if at some point nVidia decides to separate driver support for your cards. At the moment, both the 7xxx and 8xxx series are supported by the same driver, but that may not necessarily be the case in the future.
Dwarf