There are very few reasons to go with 32-bit.
Those reasons would be:
1. Your processor cannot run 64 bit ... If you are getting a new machine that shouldn't be an issue. In fact when looking for a new machine if it says 32-bit preinstalled then you should question, "Why? Is this an older model with a 32 bit processor? Or is it underpowered with only 1GB of RAM?"
2. You do not have enough RAM. 64-bit likes at least 2GB of RAM. It may run on 1GB but sluggishly and it will not have enough RAM to run Virtual PC - XP mode if you need it. With 4GB you need not worry as you are in "the sweet spot" for memory with W7.
3. You have work hardware or software that absolutely requires 32-bit and cannot be replaced or upgraded. Some USB security cards etc have issues with 64-bit and if your job requires it then ...". Also, older 16-bit software will not run on native 64-bit. Since you mention W7Pro I will add that you are entitled to download Virtual PC - XP mode for free from Microsoft and it may be able to run those older hardware and software programs in it's emulated 32-bit XP system even if they will not run in native 64-bit.
SO the question isn't really do I need 64-bit, the question is, why would you go 32-bit unless you absolutely have too.
BTW: On a 32-bit system only about 3.25GB of RAM is addressable so the other 3/4 GB goes to waste if you install 4GB.
