If I understand the info on the next link correctly, it is possible if you enable coolbits via a registry tweak. You will however be limited to the lowest cards memory amount. Anyway... read up and decide for yourself:
Considering you need an expensive motherboard to enable sli on geforce 8 cards (because u need a c2d or better to feed them data) it may seem a bit uneconomical indeed, but I'm not entirely sure it's a bad idea really.
It'd probably be better to sell them both and get a gtx, but if one just happends to have both, it may give enough performance boost to keep up with the big brother.....
do report back if you decide to try it - what's the worst that can happen? I suppose it'll just revert to single card setup if you remove a card, so u probably can't do much harm with it.
but if you aint' got both cards yet, Id not suggest investing in one more
well thats the thing with the 320 and the 640. as far as i know, they are the same card, but with diff. amounts of memory. but like the other guy said, the 640 would be limited by the 320 card. even if you were getting the 640 for free, it would be better to sell both and get a gtx card. but it sounds like the second card might not be yours to sell.
When purchasing a second graphics card, you should try to match the memory size so that you are ensured full value and performance from your purchase. However, while it is not recommended, NVIDIA does offer the flexibility to run graphics cards with different sized memory by using CoolBits. Using CoolBits (value set to 18), you can force both of the cards to use the lower of the two memory sizes and operate them together in SLI mode. When dissimilar memory sizes are enabled to work together using CoolBits, the effective memory size for each card becomes the smaller of the memory sizes.