Question about the 4870 X2

I built a computer just recently,

ASRock x58 Xtreme Mobo
Corsair CMPSU-850TX 850W PSU
Intel Core I7 920 CPU (OC'd to 3.4ghz)
12 gigs DDR3 1600 Corsair ram
WD Caviar black HD 640 Gigs
Visiontech 4870 X2 vid card

My question is with the card, first off this computer is running everything I throw at it at 1900x1000 resolution, and this computer in total cost me roughly $1600 to build, so im planning on using it at least for the next 4 years without touching it (knock on wood). I know about the crossfire technology with the ATI cards, but my question is since this is a 4870 X2, if I buy another identical card, is it possible to run those 2 in crossfire mode? In other words like running 4 gpu's? And how much benefit would I see by doing that if it is possible?

That 850w will be cutting it close at 100% load, but in theory, it should stand up to it.