Quake 4 with QuadSLI
If you're looking for a Quad SLI performance showcase, perhaps Quake 4 is it, since it is currently the only game engine we're aware of (aside from maybe the Doom3 engine) that supports Quad AFR rendering in Quad SLI configurations. More details ahead.
At 1600 and 1920 resolution the EN7950GX2 obliterates any single card score in this test and even takes on an ATI Radeon X1900 CrossFire setup--a combination that will set you back at least $300 more than the average price of one GeForce 7950 GX2 card. In fact, in Quake 4 we actually see the best scaling of performance yet for the GeForce 7950 GX2, versus the 7900 GTX SLI setup we've compared it to.
As you can see by the dual vertical load-balancing indicators in the Quake 4 screen-shot above, Quad SLI is running in Quad AFR mode, which is the most efficient method of load balancing for a pair of these cards in virtually any game engine.
The Quad SLI setup containing the Asus EN7950GX2 is some 35% faster than even a pair of Radeon X1900 cards. Unfortunately, we weren't able to test a GeForce 7900 GTX SLI combination but it would suffice to say that the performance of this pair would fall somewhere in between a GeForce 7950 GX2 and the Quad SLI pair of 7950 GX2s. A single GeForce 7950 GX2 isn't what we would call all that playable at this resolution and 8Xs AA setting but at 1600X1200 resolution you should have no issue.