8800gts 320mb + 640mb SLi?

Can be done, but the 640 will be limited to only using 320mb of memory. Similarly if any of the clocks are different between the two cards, the faster one will downclock to match the slower.

Put the 320mb card in the top PCIe slot and you will be fine.

Yep as TurboTurtle said, it will work, I SLI'd a 384mb 9600GSO and a 768MB one, both of them were limited to 384mb of RAM, but it was still far faster than a single card. All you need to do is install the cards, attach the SLI bridge, boot up, install drivers, and go to the Nvidia control panel and select "Enable SLI"

Took me a while, but I got it done. Couldn't for the life of me get the "Enable SLI" button to even show up. Tried reg-edits, switch the gpus to the alternate slot, tried drivers, wouldn't do anything. I gave up and rebooted, and low and behold the setting was enabled.

Now the next questions: how can I overclock? The coolbits reg-edit doesn't work, and rivatuner/atitools is saying that windows can't access the gpu.

Edit - Nvm about overclocking. Right now because the card are so close, my top card is hitting 103 after 10 mins of furmark. That can't be good.

google for and download evga precision, set the fan speed up at 65% or more and then move up in increments or 5-10% and test stability with something like 3d mark vantage