There are several things to look for
Graphix card.
Integrated, PCI-Express, SLI, or AGP
Number of add-ons.
For example are you going to add on a PVR
Number of USB ports
Do you want Firewire
Amount of Memorey something like - 4x 184pin DDR
means you can put 4 sticks of 184 pin DDR memorey.
Do you want DDR2 Memorey.
FSB Speed, etc...
There are a lot of different application and needs.
As far as Buy the Motherboard and build around it. I think you need to ask yourself, what are your plans? Are you going to upgrade are you going to overclock, etc.
Give the group what you want to do and what your plans are and I'm sure they can give you a better answer