Below you'll find a few benchmarks using Tri-SLI 580 at 3 x 1080p. I think it bears out my general conclusion: if you're OK with 2-4x MSAA and high-very high settings, there's no need to get 3 GB cards and 1.5 GB will suffice.
Even in the scenarios where you hit max vRAM limit, it's not choking performance, based on the FPS and my own observations watching the benchmarks. I don't know if this is because the engine is smart about it or because the extra data is offloaded to surplus RAM rather than the HDD.
There are a couple exceptions - 8x MSAA with Heaven benchmark will hard crash the machine, and according to [H], DAO2 with 8x MSAA chokes 1.5 GB vRAM - but that's outside the 2x to 4x MSAA parameters that I find sufficient. (Mainly because the IQ difference between 4 and 8x MSAA is next to meaningless, shader-heavy games have reduced the value of MSAA in the first place, and the next real IQ improvement - supersampling AA - isn't feasible at this resolution due to sheer horsepower requirements anyway).
DirectX 10 GAMER 1X @ Map: avalanche @ 0 5760 x 1080 AA 0x
==> Framerate [ Min: 59.34 Max: 115.81 Avg: 82.95 ]
DirectX 10 GAMER 1X @ Map: avalanche @ 0 5760 x 1080 AA 2x
==> Framerate [ Min: 44.75 Max: 68.35 Avg: 56.13 ]
DirectX 10 GAMER 1X @ Map: avalanche @ 0 5760 x 1080 AA 4x
 Framerate [ Min: 38.83 Max: 61.97 Avg: 49.32 ]
DirectX 10 ENTHUSIAST 1X @ Map: avalanche @ 0 5760 x 1080 AA 0x
==> Framerate [ Min: 47.68 Max: 73.60 Avg: 59.50 ]
DirectX 10 ENTHUSIAST 1X @ Map: avalanche @ 0 5760 x 1080 AA 2x
==> Framerate [ Min: 39.68 Max: 61.42 Avg: 50.83 ]
DirectX 10 ENTHUSIAST 1X @ Map: avalanche @ 0 5760 x 1080 AA 4x
==> Framerate [ Min: 36.82 Max: 59.75 Avg: 45.77 ] 1512 MB max vRAM
JUST CAUSE 2 Max settings + 4x MSAA, 39.2 FPS AVG, 1112 MB max Vram
ARKHAM Max settings + normal physx + 4x MSAA: 117 FPS, 1527 MB max vRAM
HEAVEN 2.1 Regular settings + 16x AF + 4x MSAA: 53.6 FPS, 1522 MB max vRAM
BAD COMPANY 2 Max settings + 4x MSAA MP MAP HEAVY METAL: 1250 MB max RAM
Note: I didn't bother running with 3DVision because Nvidia software appears to impose a 2x AA limit when running 3D and Surround. It's not clear to me that it succeeds in doing so all the time, so it's an unnecessary variable. 3D does impose a 10-20% vRAM overhead, but that may or may not be canceled out by running at 2x AA and the fact that you can't notice aliasing as much with 3D on.
Even in the scenarios where you hit max vRAM limit, it's not choking performance, based on the FPS and my own observations watching the benchmarks. I don't know if this is because the engine is smart about it or because the extra data is offloaded to surplus RAM rather than the HDD.
There are a couple exceptions - 8x MSAA with Heaven benchmark will hard crash the machine, and according to [H], DAO2 with 8x MSAA chokes 1.5 GB vRAM - but that's outside the 2x to 4x MSAA parameters that I find sufficient. (Mainly because the IQ difference between 4 and 8x MSAA is next to meaningless, shader-heavy games have reduced the value of MSAA in the first place, and the next real IQ improvement - supersampling AA - isn't feasible at this resolution due to sheer horsepower requirements anyway).
DirectX 10 GAMER 1X @ Map: avalanche @ 0 5760 x 1080 AA 0x
==> Framerate [ Min: 59.34 Max: 115.81 Avg: 82.95 ]
DirectX 10 GAMER 1X @ Map: avalanche @ 0 5760 x 1080 AA 2x
==> Framerate [ Min: 44.75 Max: 68.35 Avg: 56.13 ]
DirectX 10 GAMER 1X @ Map: avalanche @ 0 5760 x 1080 AA 4x
 Framerate [ Min: 38.83 Max: 61.97 Avg: 49.32 ]
DirectX 10 ENTHUSIAST 1X @ Map: avalanche @ 0 5760 x 1080 AA 0x
==> Framerate [ Min: 47.68 Max: 73.60 Avg: 59.50 ]
DirectX 10 ENTHUSIAST 1X @ Map: avalanche @ 0 5760 x 1080 AA 2x
==> Framerate [ Min: 39.68 Max: 61.42 Avg: 50.83 ]
DirectX 10 ENTHUSIAST 1X @ Map: avalanche @ 0 5760 x 1080 AA 4x
==> Framerate [ Min: 36.82 Max: 59.75 Avg: 45.77 ] 1512 MB max vRAM
JUST CAUSE 2 Max settings + 4x MSAA, 39.2 FPS AVG, 1112 MB max Vram
ARKHAM Max settings + normal physx + 4x MSAA: 117 FPS, 1527 MB max vRAM
HEAVEN 2.1 Regular settings + 16x AF + 4x MSAA: 53.6 FPS, 1522 MB max vRAM
BAD COMPANY 2 Max settings + 4x MSAA MP MAP HEAVY METAL: 1250 MB max RAM
Note: I didn't bother running with 3DVision because Nvidia software appears to impose a 2x AA limit when running 3D and Surround. It's not clear to me that it succeeds in doing so all the time, so it's an unnecessary variable. 3D does impose a 10-20% vRAM overhead, but that may or may not be canceled out by running at 2x AA and the fact that you can't notice aliasing as much with 3D on.