The dual processor quad-core macs are pretty impossible to beat right now, if you have an app that can make use of more than 4 cores (any of the "big boy" apps).  My personal preference is an Intel running Microsoft, but it's pretty hard to find a dual socket 775 motherboard (I'm unable to find ANY), so if your apps support that much hardware, the top shelf Macs are the penultimate.

What I have currently, and imo pretty close to optimal for the Intel world:
Intel QX6700 (the more recent QX6800 marginally beats it, but both are very overpriced next to the Q6700 at around half the cost)
Asus P5WDH Deluxe motherboard
8GB memory
4x 500GB SATA hard drives, with 3x in a RAID0 array (striping, no parity) and 1x off to the side for backups and miscellaneous stuff
NVidia 8800GTX, 768MB video memory
20" NEC LCD monitor
A too-damn-big Wacom Intuos3 (I think 6x9) with airbrush thingy
A Genuine Clickety IBM Keyboard (the best part of all)
Windows XP Pro 64-bit edition (extremely bulletproof and well-behaved)

My Freebies