You should get the triple-channel kit since the LGA 1366 chips have a triple-channel RAM controller. You'll get the best performance with the triple-channel kit. You should consider going with a 6GB kit if you're using a 64-bit OS.
Yes. You can surely add a second set for 6 gigs:-) Probably the best choice if you are tight on cash.
Edit- very solid selection, though I wouldn't quite call that a full build, without a case, PSU, GPU, HDDs etc:-) Should get you a very nice system though.
Yes, you can do that. It's much better to just start off with 6GB if at all possible since running a motherboard with all the RAM slots filled up can cause stability issues. If going with 6GB off the bat isn't an option then you can add another 3GB later.
Right- you can, but it won't perform as well as triple. Really, it doesn't make sense to go double channel on a triple channel board. Its better to go 3 gigs triple channel than 4gigs Dual Channel, and they're similar prices.