To make the 2 9800GT's work in SLI you would have to force the one 1GB card to be 512mb to match the first card. So you would be losing half your memory on one card. plus the fact that the memory probably runs slower on the 1gb card means that the original 512mb card's memory clock will underclock to that of the 1gb cards memory clock. Im not 100% sure about putting an 8800gt and a 9800gt in SLI. The only big difference between them (hybrid power doesnt matter) is the fact that the 9800gt uses a 55nm process rather than the 8800gt's 65nm process.
Bottom line - For SLI to work, the 2 cards have to have identical bios's.
Good Luck...