With the card you have, i wouldnt get a normal 5850 or 5870 to crossfire with it, as you loose a big advantage of that card. In CF, cards only use the size frame buffer as the smallest in the setup, so if you pair your 2GB 5870 with a 1GB 5870 it will act like to 1GB 5870s in CF, since you got the eyefinity edition i assume you have multiple high resolution monitors correct? If so then the extra GB of VRAM will be helping you out especially in CF when you have more processing power to make use of it.
But yes, you could CF it with a 5850 if you wanted to, it wouldnt boost performance quite as much as getting a second 2GB 5870 but it would still give you a boost.