Ahem....SLI will start to KICK ARSE as NVIDIA works on their drivers. In high res and FULL AA ANIS a single card is reduced to nearly HALF that of SLI setup in some games. Buy a new SLI CERT MOBO like JoshSB said and get another 7800GT when the price drops a bit more. Also get a new SLI PSU. The gains of SLI will grow tremendously and a 7800GT is a great place to start. I've seen benchmark after benchmark and the 7800GT SLI really pulls when things get rough! But yes game support is key, but that will change with time.
Its really funny though. Your system is almost identical in every part as the one I plan to build.