alot of companies are starting to support 64 bit vist/windows 7 so that's an option. Of course mac is always an option too, you know most apps support mac.
I don't think there is a "best" I think there is a "what works best for you and your budget"
After much deliberation, I'm probably going to build my own pc for my music stuff, even though many programs run on 64 bit windows, most still don't take advantage of the extra ram addressing available so it doesn't really matter.
4gb of ram is still good, and powerful enough to run many studios. Unless you're running tons and tons of fx all the time, but getting a good dsp/pci card can help with that.
As far as the brands, they are all pretty much the same now days, dell, hp, sony, acer, etc...all good and have some preconfigured machines that are pretty powerful.