I do 30s 1280x720 spots every day on my i5 iMac (16 gigs RAM) - I have multiple layers from motion and I'm usually humming right along in ProRes 422.
I have done longer form stuff with lots of layers where I feel it bog down a bit and I wish I had more horsepower - but I seem to remember feeling that way with every system once it reaches it limits - I guess you'd just reach that boundary a little later with a top of the line mac pro -
obviously the budget is the big concern (was for me anyway) but depending on what your doing, the iMac ain't no slouch
HOWEVER - if you are looking to set up an ethernet SAN - DON'T EVEN THINK ABOUT A NEW iMAC -
the ethernet controllers on i5 and i7's don't allow jumbo frames - so you will be bummed out there (and no fibre channel of course)
it's a solo sneakernet station