Preliminary benchmarks
NOTE: The following graphs start at 0.6 instead of 0, this makes the the performance increase look much bigger than it really is. Also some games are not available in one graph, but used in another.
While some users still have a 17 or 19 inch LCD, others will still use old monitors or 20.1 inch LCD capable of 1600x1200 resolution. This is definitely where the performance gain can be seen. In other words, if you are still running a monitor with a maximum resolution of 1280x1024 you will most likely be limited by your CPU instead of your graphic card. Here we can see that certain games have a big spike in performance. The highest gain is seen in FarCry and in F.E.A.R.
Having a monitor capable of running such resolution is quite rare. The normal gamer does not have a Dell 3007WFP standing at home, not even a 24 inch LCD. But this is what HD gaming is about. Even consoles are pushing HD gaming these days. At 2560x1600 Doom 3 gains the most in a single card configuration. Call of Duty 2, Quake 4 and Half-Life 2 get a very healthy boost in frame rates as well.
While QuadSLI will not be available right away to the general DIY gamer, system builders still offer these setups. Running QuadSLI compared to two 7900GTX in SLI gives the gamer a huge bump in Painkiller at 2560x1600 with 8xSLIAA & 16x AF. On the other hand, some games like FEAR, Oblivion (outdoor environment), Half-Life 2 and Serious Sam 2 only get a 10 to 20% boost in performance.