In games that support CrossFire (I'm not sure which don't) the 4890s will undoubtedly perform better. In games that do not support CrossFire, the 285 will perform better.
The 285 will draw less power, make less noise, and produce less heat.
A single 4890 will run pretty much any game right now at max settings at 1920x1200 without breaking a sweat.
If possible, wait until ATI releases their DX11 cards later in the year.
The crossfire will utterly flatten the 285 whenever crossfire is supported (which is a decent chunk of the time). The rest of the time, the 285 will be slightly ahead, though you can get a 4890 to nearly equal a 285 if you overclock it (and have a fairly lucky card).
------------------------------Asus P6T deluxe
i7 965 @ 4.2GHz (200*21), 1.384V
12GB Corsair Dominator DDR3-1600 CAS 7
Reply to cjl
Yah, i was going to overclock the 4890s to 925 - 950 mhz (from 850 mhz) as i will have 2 giant 240mm fans, a 120m mm fan, and a 240mm liquid cooled radiator. Also planning on OC a 2.66 ghz i7 920 to 3.8 ghz or 4.0 ghz.