It really depends on your processor. If it supports 64 bit instructions (pretty much any desktop processor built and sold in the last years; as for laptops/Macbooks it really depends on the CPU you've got), go with any 64 bit version of Windows 7. I wouldn't wait for Windows 8. I don't expect the price to drop significantly, and even then I'm not really sold on the whole Metro/touch experience being forced on non-touch devices (i.e. most desktop PCs).
I'm rather sure non-OEM copies of Windows 7 include both editions, 32 bit and 64 bit, so you shouldn't be able to go wrong here. It won't allow you to install 64 bit, if your system doesn't support it.
Edit: To check your CPU (to see the whether it supports 64 bit or not) check this link.
Edit 2: Here's a (incomplete?) list of Macs and the verisons of Windows 7 they support.