If you aren't overclocking (and I hope you aren't doing serious work on an overclocked machine) the only spec relevant to you is the volume. The best price/performance right now is the 3x2 GB and you won't be able to fill these anyway except with some very specific applications most people don't use (creating big 3D models, or some serious video editing). 4 GB modules are too expensive anyway.
If money is no objection, get memory from a known brand, as I've known people who had stability problems with no-name RAM.
My personal experience: I have the Core i7 930 with 3x2. Because I went SSD only, I pushed /tmp into RAM and have no swap. I've never run out of memory, typical usage is about 1.5 GB with 10+ applications running.
Another thing: most shops will sell you a machine of this class with 1600er RAM although the 930 only supports 1066 per spec. There is no trouble with that, as the price is about the same, you don't need extra cooling, it runs as stable as at 1066, and the warranty is still OK. On the other hand, the speed of RAM is rarely a bottleneck, so you don't gain much performance. Just take whatever speed they offer you, there isn't much difference.
You'll probably also see listings of memory giving latency numbers. They're only interesting for overclockers, just disregard them.
