If you wanna continue using Shake Id recommend using a Mac as they still support that OS.

At work I use a quad processor MAC with a couple gig of Ram, not too sure of the exact specs but she runs like a dream.

If you wanna work on windows you'll be stuck with using Shake 2.5 (the highest version out there for windows) but for either windows or linux OS I tend to stick to a Dell or Alienware workstation. The Dell precision models tend to be pretty stable and if you get enough RAM and a large enough processor (ideally dual but a super fast single will work too) then your all set.

Be sure to invest in a good quality wacom tablet.

And seriously please be sure to use a legit version of shake as the license is only like $900 or 400 pounds these days for the Mac OS version so its really worth it. Its still industry standard (In feature film anyhow).