Sure 2 cards for $500 to get near a single card solution that costs $600 (Probably will drop fast) and sometimes beats it isn't a bad idea. Of course you have to have a more expensive
motherboardhttp://en.wikipedia.org/wiki/Motherboard with Crossfire support, you also need a more powerful power supply to handle the additional cards. Your computer will then draw more power, costing you more money on your electricity. Your case will also run hotter, possibly hurting and OCing performance on your CPU or GPUs.
So actually you are going to spend just as much money on your Crossfire solution when a single card solution is just simpler. For an extra $50 get a good aftermarket cooler on your single card solution and OC it.
And once these cards launch and nVidia sees that the ATI cards are pretty close to the GT200 cards, they prices will fall accordingly.