Windows 32-bit will be comptible with a 4GB RAM installation but it won't see the whole thing. You'll get lucky to see 3.5GB of it. It's fine though I think. That's why I just got 3GB instead. Cheaper
You have to be careful with Windows Vista x64. Office 2007 is supported under that so no problems there. You'll need to check all the software you use to see if there are compatibiltiy issues, including drivers. Go to each manufacturers page to see if there are x64 drivers. If not, forget the x64 upgrade and stick with regular Vista 32-bit.
Are you looking to spend much on the motherboard? I don't know what your budget is, but here's a simple motherboard that meets your needs:
ABIT IP35-E LGA 775 Intel P35 ATX Intel Motherboard - Retail
Abit is one of the premier manufacturer and that is a good looking board. Not top of the line, but it's a great performer. It's simple, easy, and right to the point motherboard. No hassle. Plus, it's under $100.
I see you also have the DDR21000 RAM. If not OCing and you are on a budget, you should just get a cheaper DDR2800 RAM from G.Skill. If you
Just buy two of these babies:
G.SKILL 2GB (2 x 1GB) 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400) Dual Channel Kit Desktop Memory - Retail
Get this free w/ AMD combo, limited time offer
..........and get a total of 4GB. You'll love it. I have those exact same RAM, but DDR models. Great bang for the buck.