Some thoughts on this question.
I'm using a Mac Graphite Computer; does have dual processing, but is limited to a speed of 1400 Mhz. Slow, indeed very slow by todays standards. Even so, it's quite capable of editing large images, and processor intensive filters quite fast. Normal processing, such as contrast and color adjustments, or unsharp mask, are almost transparently fast. I have to wait about five seconds for it to use a filter like Noise Ninja.
Why not buy a Mac Mini? They are cheap, and would leave you the cash to buy that good monitor and other peripherals that you desire? You can run the Mac OS, or Windows and linux. They're tiny, easy to install, user friendly and reliable?
Certainly when my machine hits the dust, that's what I'll be using.
As a semi-professional, I don't find my present machine a serious limitation. The new Mac Mini's are far faster.
I believe there is too much emphasis are getting the most powerful machine, when in fact most of what you will be doing, simply doesn't require that kind of speed.
Just some thoughts on this question.