One of my many jobs is selling technology (sadly, the one I'm salaried for, currently), and here's the rundown:
Vista Home Basic is pretty crippled.
Vista Home Premium is what I would take as a baseline at this point. It's full-featured, and though you'd never use the Media Center functionality on a dedicated recording machine, it's the cheapest version that's got everything you want.
Vista Business is overkill. It's basically Home Premium with more support for complex networking, domain access, and encryption. Some people need those things; I'd wager you probably don't.
Vista Ultimate combines the features of Business and Home Premium. You really won't get any better performance, and the extras are a joke.
If you're going to run Vista at all, go with Home Premium for a stand-alone recording/mixing/editing box.
Really, though, I have Vista on one machine and XP on two computers, and I do all my recording with XP. No matter what you do to Vista, you'll never get it to use less resources than a finely-tuned installation of XP professional. Once you get all the unnecessary services turned off, the page file turned off, etc. etc. it'll run beautifully and be remarkably stable unless your hardware goes bad (the one thing that ISN'T Microsoft's fault).
Hope that helps.