I would look into X48 motherboards. Price is higher though.
GIGABYTE GA-X48-DS4 LGA 775 Intel X48 ATX Intel Motherboard - Retail http://www.newegg.com/Product/Product.aspx?Item=N82E16813128336
There's also a P5E Deluxe, a few bucks cheaper than the Gigabyte. Personally I'd get the Gigabyte, but maybe you prefer Asus.
Edit: there are some cheaper motherboards based on P45, which support Crossfire too, but they're not a good idea. Example P5Q or GA-P45-DQ6. You should pick X38 or X48. This article explains it.http://www.tweaktown.com/articles/1472/intel_p45_vs_x48_crossfire_performance/index.html