In order for SLI to work, both cards must have the same information in their RAMs. So if you SLI a 1GB with a 512MB card it will probably work, but you will get the performance of SLIing 2 512mb GTS 250 cards
"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 RAM to 256MB to operate with the other card, so it wont have its true power. (This option isn't recommended. )"