i5, i7 or Nehalem for heavy Illustrator use?

Hi, I'm new here and I would appreciate your help. I am using Adobe Illustrator quite intensely on a pretty basic (sad) HP laptop (Intel Core 2 1.67GHz / 2GB RAM) and since I have decided to upgrade to something powerful; I was wondering which Apple product would be more suitable for me. Although I have tried researching about this, the only answers I've received are benchmark results regarding performance running Adobe Photoshop and since I only use it mildly I can't really compare it to Illustrator which I use it to edit files up to 60MB in size (Although they are bound to become much larger with a more powerful machine -150MB+). Also, regarding the processor; Would I be needing something more powerful than the IMac i5? If yes, I can go for the i7 IMac or even the Quad-Core Mac Pro, but I would really appreciate some help clearing out whether I'll need an i5, i7 or a Nehalem processor to run Illustrator smoothly with heavy use (and also how many cores does Illustrator utilizes).

Thank you in advance,
Marcos