You may want to consider a Mac Pro tower. It has everything you want and will run Windows fine. I have a buddy who has one and runs Vista on it 95% of the time.
I am sure the Mac Pro is an excellent computer but one of the main advantages of building a computer is to get exactly what you want. For instance the Mac Pro does not support SLI dual graphics cards. This is something mostly used by gamers right now. But my understanding is that future versions of Photoshop will make use of the processing power in the graphics card (some programs do this now.) From the little I've heard it seems that the high end graphics will make a big difference on the next Photoshop - CS4. So you may want a motherboard that supports SLI graphics even if you only buy one SLI graphics card to start. The newsest SLI boards can support three SLI cards (only certain model cards can be ganged in three.) The Mac motherboard requires a RAID card whereas lots of PC motherboards have RAID built in. Various motherboards have special features including overclocking capabilities and software to adjust and monitor the board.
I am not sure if you can get a dual processor SLI board. If not, it might be a good idea to wait until you can find out what features in a computer will do the most to accelerate Photoshop CS4. (Processor, memory, graphics, or 64 bit) That is what I am going to do.
There is no word from Adobe that this GPU acceleration will be in CS4. But NVIDIA makes SLI motherboards and graphics cards.
There is some controvery surrounding this report:
In addition, the Mac Pro is limited to 4 internal hard drives and what look like 2 5 1/4 bays. My PC case has space for 6 3.5 inch hard drives (all shock mounted on silicone bumpers), a 3.5 inch front accessible slot for floppy or flash card reader or whatever, and 4 5 1/4 bays. I currently have 5 hard drives installed and probably will add one more. Additionally, my case has three large fans and room for more. The hard drives are mounted vertically with space between them for cooling. There is enough room for separate hard drive fans if I find that necessary. (I am going for a very robust system.)