There is a lot of assumptions here, since there are very few benchmarks directly comparing those two very rare setups. Performance depends on the specific game. Here are some from guru3d, including quad 9800gx2, tri gtx280, and quad 4870x2.
Tri-SLI'd GTX 280s is one of the 2 highest performing GPU setups available. 2 9800 GX2 doesn't even beat ONE 4870 X2.