Something doesn't quite make sense to me. Why did you buy a triple channel DDR3 kit, but a socket 775 CPU? Based on your GPU selection, I presume that you're not a gamer.
Most socket 775 motherboards support DDR2, not DDR3. Here are a few motherboards:
ASUS P5Q3 DELUXE
Personally I wouldn't buy any of the above. I'd buy DDR2 memory and a Gigabyte GA-EP45-UD3R or GA-EP45-UD3P if Crossfire eventually is required.