I personally can't see any reason to get a macpro, if you wanted to edit in final cut sure, but you're one of those adobe people (cough)
Buy a PC... Gigabyte i7 mainboard, intel 20 chip, 6gb of good ddr3 ram, ATI HD 4870 video card, Raptor 300 GB HD, Samsung 1TB HD, corsair HX 650 PSU, Lian Li case.... if you want anymore details pm me
Sort of out of thread but will get back, why would you go with an HD 4870 instead of two 4850s in Crossfire?
To the OP, as someone else said, you may want to wait for the Nehalem Pros. Also look to the other point about graphics cards, the Apple ones do seem to be quite bit behind the times, but still adequate for most purposes. Having said that, though, if you think you will want to run some Mac specific software now or in the future, the Mac Pros are quite a decent deal for the hardware in them and you can always install Windows on them for the software you need that only runs on that OS.
Switching from Windows to OS X will also take a bit to learn as well, though not incredibly difficult. Just realized I'm typing this post up from Windows Vista - I switch a lot between XP, Vista and OS X for various reasons and have just recently put Vista on my Macbook Pro, I used to have to switch machines all the time. The ability to swap between OSs on the same machine is very nice.