What should I buy?

I've been editing professionally for about 5 years. I have been using Final Cut on a MacbookPro. I am looking to upgrade in order to have something faster and more powerful. I see Apple just came out with a new iMac. Would something like that be powerful enough? Or would you recommend a Mac Pro?

Also, would you recommend buying new or purchasing a used computer?

Thank you very much!

Clarify, is this to use kdenlive or FCP? If it's for FCP then this forum really isn't the place to be asking.

Info : I have just "open" a MacBookPro 10,1 Lion, to Ubuntu 12.4 (dualboot), and directly installed Kdenlive !!! Seems to work like a charm.