Almost identical. It depends on the game:http://www.tomshardware.com/reviews/radeon-geforce-roundup,2297-17.html
If you get the GTX285 you have some advantages for applications that use CUDA or physics that the ATI cards won't give you.
The main advantage of the dual 4890's is you should be able to reduce the noise slightly.
I'd be tempted to go with the GTX285 but look for one with a good, quiet cooling solution.