Well two GTX 460 1gb cards will be better than crossfired HD5850s and cost less if you can sell the HD5850 for a decent price.
You're not happy with your 5850 performance?
Good Question !
EDIT :-- What games are you playing at what RES ?
Are you an enthusiast? To me, HD5850 is already strong enough to play any games recently with decent setting.
ya i am an enthusisast i play games on a 23" LG 1080p intel i7 im always lookin to up grade i guessi always want more lol i play dragon age bad company 2 and so on so really anything the 5850 does great by its self jus always want to look at more power