Choose the chipset first. I went to P45 because it's less power-hungry than nVidia and its process is more recent than X48, so it boasts real overclocking potential - I still haven't raised voltages and already run at +33% Cpu and Ram).
Then, I chose ddr2 because the P45 then allows 16GB instead of 8GB with ddr3 - not brilliant, is it? And not even on every mobo!
And then, decide how many Pci-E you want, and whether two wide ones - and choice gets narrow enough.