I was stuck with this same conundrum 6 months ago: Need a computer, don't want vista, need windows for compatibility. I was going around all the shops looking for a small nifty laptop and as I went down the spec-list for each prospect my face would always fall where it said windows vista.
I believe the recent service-pack for vista fixed a lot of the issues, but at the time I knew from personal experience and that of my friends, that vista would take any nice hardware and reduce it by about 50% of what I'd get with XP. My brother, who's in Media and uses macs all the time said: 'get a mac, you can stick XP on it and you'll be set'
I went and got a beefy macbook, setup parallels with an old copy of XP I've had lying around and hey presto! I've even discovered that the compatibility problems weren't as pronounced as I thought and I've rarely had to boot into XP at all!
I reckon you might pay a premium of about 10-15% over standard high-end PC hardware, but for a completely integrated solution, with well-tested hardware, reliablility and no *major* compatibility issues, that's money well spent in my (mac) book!
(my only regret at this stage is that I didn't go the whole hog on a macbook pro. The standard macbook, tho beautifully designed, is a little 'girly' looking, and I cant help but feel a little self-conscious in meetings and on public transport).