OEM pack 64 or 32 bit W7
I was told at the shop that, provided you have a legit key, you can install either 32 or 64 bit with that key. You just have to have the appropriate DVD to do it (or are there ISOs for 32 bit and 64 bit on the web?).
I'll let you know in a couple of weeks; I installed 32bit W7 and am about to swap to 64bit.