I performed a ton of tests at 1050P using 16xAA/16xQAA with extreme IQ settings. If your interested. They show single GTX 260/280/295, And 260 SLI/280 SLI, and 295 Quad SLI.
I know its not the most extreme resolution ((was having monitor problems at the time)) but the performance advantage only goes up until you reach 2560x1600 with 8x MS. Might be worth taking a look at. If your gaming at 1680x1050 without transparency SS and low 4xAA you probably wont see a difference in the majority of your games though.