It depends what standard of memory the motherboard itself supports. I'd recommend DDR2-667, it's fast and cheaper, but if you can get DDR2-800, get that.
CrossFire is getting completely revamped when R600 comes out. No more cables or anything, it uses the SLI form of connecting the cards together. A bridge. Which should increase bandwidth and become much faster. if you got any other CrossFire questions, feel free to ask.