The best Motherboard companies include:
In order from best to worst (in my opinion)
DFI
MSI
ASUS
Abit
Gigabyte
Things to look for that make it more expensive are
Does it support SATA2?
Does it support Athlon X2s?
Does it support SLi? (Not so important)
Other things you should also make sure it does is
Support socket 939
Support DDR400
Have an nForce4 chipset
Support ATX (if you have an ATX case of course)
Support Raid 0,1,0+1
Have onboard audio
Have onboard LAN
Of course from all this you can't determine some things such as how good the board is for overclocking so you should read up some reviews on the board before you buy it