First up you can install windowsXP and windows64 on a MacPro, so if you do need some PC software you have plenty of options - as far as compatibility.. its windows.. what runs on windows runs on windows, with Macpros thier almost entirely "PC" based hardware now so windows runs under its own steam not under emulation.
The MacPro is by far the best option for video editing. Its used by 90% of professionals together with Final Cut Pro. Anyone here thinking the PC can compete (windows i mean) with a Mac for video editing hasnt used it under a full commercial production. We use Macs for our Compositing and Editing for the past 5yrs (as long as the company has been going) and the users have been using them for longer again.
They are Quite solid machines, no less stable than windows (of note thier just as "unstable" too).
The biggest pull for people atm is the cost, MacPros are very competitively priced for the hardware.
One major consideration is Ram cost, because these machines are using the new Xeons they all rely on FB ram which irrelevant where you get it from.. is very expensive (also very slow) but with video editing ram speed is of less importance to HDD raid speed.
They also have a massive upgrade path, unlike a lot of PCs (unless you consider some of the Dell servers) they have a good 8 slots for ram, so depending on budget you can go from 4 through to 32gb of ram (and the OS fully supports this amount).
The drawbacks to getting a mac, is forget games.. yes you can dual boot to windows and yes all games are compatible doing so.. but for the moment atleast the fastest card available with a MacPro is a 1900xt, which depending on your gaming requirements might be a bit slow. (you can technically use a PC gfx card but theres numerous boot issues and random issue - i wouldnt advise this)
Also you have to consider Software costs. You have to buy ALL mac software - irrelevant to whether you condone pirate software, for the PC atleast you can aquire almost any software to "test" and then purchase, forget this notion on a mac (at the least you chances are limited).
I personaly HATE macs, i hate the cheesy GUI. the spinning ball of doom (which replaces the hourglass in windows) and the insanity of single button mice (macPros have the "mighty mouse" which is 2button + scroll) I also hate the BS that surrounds mac and mac users.
Sadly Apple are now competitive, have Good hardware and solid software - for professional video editing there is no other choice.
Cheers
ROfu
PS - thier also painfully easy to use. maybe one day ol bill gates will steal some of the usefull bits from the MacOS not just half inch the pretty colors.