One of the things that people who invest in the highest end multiGPU systems get is more longevity. Gamers with 3-way GTX 285 hardware will be able to go quite a while without upgrading.
The AMD Radeon HD 4870 1GB Quad CrossFire leads in our benchmarks when focusing on the resolution that matters for this hardware (2560x1600). The 3-way GTX 280/285 leads the 4-way GTX 295 in half our tests: it's a wash and it's either slightly cheaper or slightly more expensive depending on the specific flavor. The 4-way Radeon HD 4870 1GB only leads 3-way GTX 280/285 in 2 out of the 6 tests, though it ties in one of them (Fallout 3 again). If GRID were added back in, it's likely the playing field would be completely even on that count.
If you don't happen to have a motherboard that supports 3x double-slot x16 physical PCIe cards, 4-way will have to be the option. In that case, Quad 4870 1GB scores points for keeping up with the Joneses, scaling, and bang for the buck.