The demo Tim gave during his public speech was running on a standard GeForce 6800 Ultra. The demos we gave behind closed doors, which included more than what was shown in public, were run on dual 512Mb GeForce 6800 Ultras using SLI. We chose to run at 1280x1024 because (a) that's the native resolution of the projector we rented and the tools demos always look way (because the fonts are crisper) when run at the projector's native resolution and (b) because SLI is friggin fast we could get excellent frame rates at high resolutions even through we've done very little optmization work.
Even with this early UNOPTIMIZED version of the engine we do these same demos on a Shuttle PC we carry around when we visit prospective licensees and they run just fine on a single 6800GT with an Athlon 64 3400 at 1024x768. A 6600GT could run these just as well at 800x600. A 6600 GT card will be pretty cheap by the time we ship a game with this tech.