To me the choice between 64 or 32 bit is a moot point, since there’s no difference in cost. Always go with the better option, yeah? You have access to your full 4 GB, and you can get the 64 bit (and more efficient) versions of applications when given the choice.
The only reason not to is if you’re dependant on software which isn’t compatible with 64-bit Windows. Companies using archaic systems for example. Never had a problem myself with such an incompatability.
That said, if you’ve waited all this time to upgrade, perhaps you can hold out for several months longer, and get [Windows 8]? System requirements should be similar. Especially if you haven’t run into memory problems so far, then I don’t think you’ll miss the 0,5 GB before you upgrade.
By the way, on my computer, Windows 7 is running a little over 2 GB. 1 GB seems to be cached, not sure what it means but I vaguely recall something about Windows reserving memory for much used applications and services. I’m using aero, and pretty much all the other performance options are checked. On the other hand, I’m not using system restore (don’t know how much that affects memory), and no virtual memory.