I'm pretty sure as long as the card has the same architecture you can SLI with it. For example the g92 arhcitecture is still used in the 9800 gt or gtx, so you could SLI with that I believe. Of course the performance will be downgraded in the 9800 to the 8800
edit: I looked it up, the 8800 gtx runs a g80 core, while the 8800GT and the 8800GTS run a g92 core. So I think you could do it.