If you need a PC, get a PC
There is a Mac version of Cubase and a MBP would have power to burn. A fast external drive with sATA or FW800 is strongly recommended for anything more than about 8 digital audio tracks. You would also want an external ADC/DAC. Obviously, depending on the number of effects, tracks and so forth, you may saturate the performance of a single core2duo chip. This is why Mac Pros often sit in studios, bristling with processor cores. How well Cubase utilises all those cores I don't know.
The sticky question is running windows on a mac, if you're wanting to use your windows version of Cubase or Sonar (windows only). I've used Vista on my little MacBook, both in VMWare and Bootcamp. Not super impressed. For audio and virtualisation, forget it. Bootcamp is pretty good, but there are driver quirks that can be annoying. The one I'm stuck with is erratic behaviour of scrolling and the right-click solution. My view is running windows on a mac is good if you have one or two legacy programs you need during the switching. However, if windows is your core application set and you are not planning on migrating to a primarily mac system, buy a Dell or Toshiba. Buying a Mac for a pretty Windows Machine is not an optimal choice on either front.
For what it's worth, most people find migrating from Cubase to the new Logic 8 a pleasure rather than a pain, and it is such good value I've known hardened pirates to buy it for the convenience of manuals and software updates.
