If you make Powerpoint presentations, 2007 will blow all other versions of Office out of the water. It just looks and works much better.
As far as other apps, I actually like the ribbon menus much more than the old menu. As soon as you get used to it, it's great to use. Memory wise, Outlook tends to be heavy on mem, but I only use it at work. Either way, it runs fine. I mean, I can even run Office 07 apps on my netbook (1.2ghz Via C-7, 1gb DDR2 533mhz) and it's fine.