I have seen strange dedicated offer from hetzner datacenter. They sell i7 920 servers with 8GB RAM.
As far I know i7 working with 3 channel memory.
How it possible to use 8 GB? It must be 6 or 9.
I have asked support but have not received exactly answer yet...
If I was you Id make sure it is the 1333mhz dual channel ram instead of some regular 2gb sticks. It would be a waste IMO not to get the full potential out of that cpu and board. I just got me one the other week and its awesome. Probably the fastest server I ever had. I got the 920 with 6 gb ram.
I have read motherboard documntation - it really works in triple channel mode with such RAM configuration.
I have not found performance comparison of 3-DIMM and 4-DIMM systems. I think speed degradation possible but will not significant - additional 2 GB RAM give to system much more performance.