Hi,  
 
I was thinking of going sli and while looking at the prices saw that the 8800 320mb gts is cheaper than the 8800 640mb gts. Can I hook these two up in SLI? Will that work or do they have to be exactly the same?  
 
Thanks
 
Here they are:
 
8800 320mb gts:
 
http://www.newegg.com/Product/Prod [...] 6814130082
 
8800 640mb gts:
 
http://www.newegg.com/Product/Prod [...] 6814130327

In theory 'yes' you can, but you will end up with two 320s.

to rayn:
http://www.tomshardware.co.uk/foru [...] sfire-faqs
 
Do the cards have to be same model?  
I answer it with an example:  
You have a NVIDIA Geforce 7800GTX 256, and you want to add another card , the second card  has to be 7800GTX exactly.  
 
But what about the memory? Can you add a 7800GTX 512 and use that with 7800GTX 256?  
It's recommended to use a 7800GTX 256 with a 7800GTX 256 and 7800GTX 512 with 7800GTX 512.  
However you can use a 7800GTX 256 with 7800GTX 512 , BUT the 7800GTX 512 will lower its clock to operate with the 256 one , so it wont have its true power. (This option isn't recommended )