I'm trying to decide the same thing. I've converted all my projects over to AC15. For my machine I'm running Lion on a spiffy new 17" MacBook Pro i7 with 8Gb of RAM. It is great - very responsive for a 44-unit 5-story building. It outpaces all out quad-core MacPro's, the newest of which is a little over 2 years old.
I've come up against this dilemma before, and tried G5 iMacs when their specs were awfully close to the G5 towers, plus you got a newfangled LCD display to boot! Ah the good old days when we were switching from CRT's! They were good for a little while, but with their limitations in upgrading memory, we phased those out sooner than I would have liked.
Now things are different. The iMac is based on the Intel i7 processor (at least in the 27-incher), like my MBP, but running at 3.1 GHz (vs. 2.3), and can support additional displays. And it can take a whopping 16 Gb of RAM.
A maxed-out 27-inch iMac with 16 Gb of RAM, with upgraded graphics card, is $2,900 vs. $4,450 for the Mac Pro with dual Xeon "Westmere" chips at 2.3 GHz and 16 Gb of RAM. That's $1,550 in the bank, going with the iMac.
The hitch is, as usual, that the rumors have the next MacPro Xeon processor (Sandy Bridge architecture) coming out in Q4 sometime, along with Thunderbolt ports, the latest graphics cards, etc.
So I'm undecided! I'm looking for comparisons between the i7 iMac and the dual-processor Westmere MacPro in terms of 3D graphics, but what little I see talks more about games and Photoshop, and it's not clear that these have that much relevance to ArchiCAD.
Sometimes I hanker for the days when offices just had to buy a $70 parallel bar and a few more triangles and scales when a new employ started. And the only drafting learning was maybe switching from a drafting machine to a Mayline. Otherwise people didn't have to get trained in office CAD standards and layers and file translations (much less come over to ArchiCAD from Revit or VectorWorks), and could just sit down and start producing! Too bad I love my ArchiCAD so much!