What kind of setup do you want to use to play out live? Final Scratch won't chew up much resources and you'd be able to get away with a pretty cheap machine. Same story with Ableton Live, though if you're sequencing virtual instruments live you should look at something with a little more grunt. Make sure it's from a quality manufacturer though.

I'd reccomend a Mac for doing the live thing as the PC version of FS is awefully buggy, not to mention OS stability. For studio production I'd use a PC since there's gernally more software available for the platform (especially freeware stuff :) ), and a stability isn't the be and end all if you get into the habbit of hitting Ctrl-S every minute or so.