not yet.....thats why gtx285 is better performing in gta 4,
all games do not support crossfire....so one 285 boosts great performance....
and also the drivers are not polished for crossfire....usually ati has given punch to nvidia by decreasing the price.....and the performance....but still is best bang for the buck...or should i say £
Because of the way pricing works. Cheaper parts generally offer higher performance/price. It's the Law of Diminished Returns. Once you get up there, you pay more for a smaller increase.
The reason to not Crossfire is that you lose an upgrade option. Add more than 1 addtional card for a total of 2 does not increase performance very much. Therefore, the single card has the ability to be upgrade for much less than the Crossfired cards. Also, the power consumption on the single card is lower, so you save money on the power supply unit and the power bill.
Besides that, there are a lot of reasons not to buy the GTX 285. Mainly the newer cards destroying it for the same price or less.
You don't enable Crossfire in a specifc game. It's enabled by the hardware and in ATI's Catalyst Control Center software. That will make the computer recognize the two cards as a single one.