For RAM there's 3 possible configurations: Single channel (multiples of 1), Dual channel (multiples of 2), or Triple channel (multiples of 3).
For modern PC's you'll either see dual or triple, older systems used single channel. For dual channel you need identical RAM sticks in multiples of 2 (depending on how many slots per channel, usually 2 or 4 sticks for 4 or 8GB total RAM). For triple channel you need identical RAM sticks in multiples of 3 (depending on how many slots per channel, usually 3 or 6 sticks for 6 or 12GB total RAM). This is of course using 2GB sticks, but that is the most common.
AMD and LGA1156 builds use DUAL CHANNEL RAM while LGA1366 builds use TRIPLE CHANNEL RAM. If you get the i5 (which is only available in LGA1156) then you'll need a dual channel kit (2x2GB or 4x2GB). If you get an i7 LGA1156 you need dual channel and if you get an i7 LGA1366 you need triple channel.
This is the other huge difference between LGA1366 and 1156, the 1366 supports triple channel RAM while the 1156 doesn't. While the only CORE difference between the i7 1156 and 1366 is HyperThreading (which I use, it does improve performance according to most reviews), the socket limits the memory controller on 1156 as well as cuts PCI Express bandwidth by a large margin (as it does away with a "northbridge" such as the 1366 platform's X58 chip which handles PCI Express as well as the southbridge connection independently). The 1156 should work fine with PCIe graphics cards but I would think a 1366 build would be better for CrossFire/SLI or running a fast GPU and other PCIe devices like RAID controllers at the same time.