Like ScottW said you would need to find a intel chipset motherboard if you want to use crossfire.
For your price range i'd suggest an Asus p5q pro, it uses a intel p45 chipset. However though it does support crossfire with 2 pcie slots, both slots go to 8x from 16x once 2 cards are used. If you dont mind shelling out around $50 more you can get a Asus p5e or p5e deluxe, it uses the intel x38/x48 chipset which allows full use of the 16x slots.
These suggestions are for your price range, if you want top of the line crossfire performance then go with rapheonix's suggestion and get a x58 motherboard with a intel i7 processor.
Just a little warning though. If you plan to go ati, be wary that getting two 4850's in crossfire may cause you unwanted grief. The latest drivers (8.10 & 8.11) for 2 4850s in crossfire may cause your computer to go bat-shit crazy, meaning to say bsod.