A friend of mine has managed to get Vista running on an old Intel Celeron 733mhz machine, with 768mb RAM. It was running an ATi 7000 graphics card (currently lacking Vista drivers) but it managed 1024x768 (albeit at apallingly low redraw rates, and a fixed 60hz refresh rate).
I'm reliably informed the result was "usable" and "impressive", despite an untimed but "long" boot period. Office 2003 was installed but only Word and Excel were used, and while they managed, performance was sluggish at best. IE7 was used, but again, found a few seconds of lag when opening a new window.
The conclusion was that the setup was more-or-less suitable for home/light office use (word processing and using the internet), but this sluggish pace was without an Antivirus installed - which i'm sure would add a substancial drag factor onto any actions. So, overall it *works* but would only be suitable for light use - advanced users should probably use a higher end machine.
Special tThanks to Stephen Smith for the actual work of losing sporadic minutes of his life waiting for applications to load and passing his findings on to me. As a final note, he expressed that "installing took ages... over an hour" so Vista on an old celeron - only if you can't afford any better and have the patience of a saint.
It did work though! And that's what you should take away from this, that Vista on a Celeron 733mhz DID work.