If you can wait, I would actually wait for the
next Mac PRO. I'm not sure you'll see a 6-core processor in anything less than a Mac Pro for quite some time anyway.
First of all, Sandy Bridge is here NOW (or next month) and it will likely effect everything from the Mac Pro on down to the MBA next year. A mid-range quad-core SB can definitely hold it's own against today's most powerful iCore processors too. And if you don't require a souped-up GPU (such as in the MBA), you can go w/o a separate GPU and save power and probably money too, since the SB chip includes a potent GPU on the main chip itself!
I expect a lot of things to happen in '11 - not just the hot new processor, but USB 3.0 (or something better), more powerful graphics, and better game support in Lion or whatever they will be calling it.
My "game" plan is to wait until late August (tax-free weekend in NC

) and then do my planned tech refresh. I currently have a very capable MacBook Duo Core (Black - 250GB drive) from '08 with the crappy GMA GPU. I'm hoping for a new Mac Pro to run Flight Sim programs w/o having to boot-it in Win 7. That and the next gen iPad, of course.
Good luck.