First be aware if you change from 64 to 32bit it means a complete and total install: backup your ids and data with Easy Transfer (built into W7) to DVDs or another hard drive, make sure you have product keys and your email if on hard drive and get 32bit drivers for your computer hardware. Then install, restore your easy transfer, reinstall all the programs you use.
You will instantly loose .75 GB of memory with 32-bit. To see what you would gain, compare Ultimate to your Home Premium
HERE.
For me personally, I don't need Ultimate because I have no need to see my windows in up to 26 different languages nor encrypt entire drives with bitlocker. I do however use two features of Professional Virtual PC - XP mode and Group Policy Manager (not mentioned on that comparison page but useful for people who are registry tweakers). If you are currently fine w/o these features then I wouldn't loose memory for additional features you don't need. Personally, if you do need the Pro features, I see the $80 to buy an anytime upgrade to Pro a better idea than what I see as a downgrade to 32-bit (and anytime upgrades are done in-place without all the reinstall work above).
If you don't need any of the extra features of Pro or Ultimate I would definitely stay right where you are. If you do then you should weigh the effort required vs the anytime upgrade expense.
