Yeah, I'd also go for the single card solution, either a ATI 4890 or an NVidia 275
As like me working on the PC a Lot, it will be on near all the time, 2 cards create extra heat in my room I can do without with summer here
and an extra card over a year will increase running costs a fair enough amount that a single cards savings could be used to buy a better single card
oh, and maybe a little quieter too
The Asus 4890 come factory OC'd to 950 MHz I understand and is still one of the cheaper cards of that type + has overvolting OCing facilities too.
But I like the PhysX feature of the 275 Nvidia card maybe...
anyways, Good Luck!