4850x2,4870,4870x2?

Currently trying to get xfire working with 2 4850s, but at the beginning of the month, I plan on upgrading CPU to PII 240, and a new graphics card. From what I understand, I should be able to get a 4850x2 to Xfire with one of my existing 4850s, is this correct?? I would also like to know from anyone that has used on about the driver support, seems like it has gotten some bad reviews on the egg. Other options are the 4870, and the 4870x2, which would be best for games like Crysis? I've heard some argue against dual GPU saying it can actually hurt performance. I have 3 monitors, gaming on the 26" at 1920*1200. Please give any advice. I would also like to know about the size of the cards I've heard the 4850x2 is a beast, even bigger than the 4870x2. If anyone has exact length measurements in english, that would be very helpful. Thank you in advance