Yes there is a difference between a cheap SLi board and an expensive one. The top of the line SLi motherboards support faster data transfer for example, the Asus AN832 support 16x full data transfer on both PCI-e x16 for each slot totaling 32x while most other SLi motherboards support a total of 16x for both PCI-e x16 slots combined. New technology are coming out but availability is not immediate. If you want to go SLi, I like this motherboard:
I wouldnt suggest getting a DFI if you dont know much about computers. They are very complex. The evga nforce4 Sli motherboard is nice. I have one and I really like it. Its pretty cheap too.