F.Y.I. Sli / Crossfire setups for Modeling purposes are a waste of both system resources and money, they don't make up enough speed really to actually be worth the cost of purchasing two cards and a psu to run them both.
Its better to just buy one really nice card.
If you want to game, then yes it would be better, as SLI / Crossfire setups will actually divide the workload of a game, I.E. for nvidia cards, one card is dedicated to rendering the game, the other would be dedicated to calculating the Physix engine if the game comes with one...
Maya doesn't take advantage of SLI / Crossfire, unfortunately, ( unless 2011 does? )