If you’re going to be pushing a laptop hard, be prepared for the accompanying fan noise while you work.
Personally, I see no real advantage for using a laptop as a main studio machine if you need to hook up external monitor(s), hard drives, audio/video interface, etc, etc. THere’s nothing “portable” about that! Just more affordable… Yeah, the iMacs are getting powerful too, but have no where near the expandability of a Mac Pro and also tend to heat up under load. (Thunderbolt looks promising, but it’s looking like it’ll be another year or two at least before mass adoption/affordable prices. And there’s no guarantee manufacturers will even do that…)
Mac Pro will let you have tons of storage, more expandable RAM (you did say you use LOTS of VSTi, right? There’s your clue!), and has the proper cooling capabilities to keep your machine quiet even under heavy load.
But I do agree with others that a 2008 MP might be a bit long in the tooth. I’d at least get a Nehalem (2009+) processor version, which is a “newer”-gen chip which performs far better than the 2008 CPUs.