Alienware is a terrible company, and only got worse after Dell bought them out. There are all sorts of horror stories pertaining to customer service, tech support, billing, etc, on the internet. You pay through the nose for crap. They use stripped down products that carry the same name as the retail counter part.
The Mac Pro, outfitted with the 8800GT for OSX, is more than enough for OSX. For windows, just drop in the most expensive card you can find connect the power cable to it from the CD drive. It uses PCI-E x16 2.0, which is the newest standard.
Get 8GB of RAM from OWC, drop in a second graphics card, bootcamp Vista 64, and go.
If you just want a PC, just build you own. I built one 9 months ago. The same Alienware configuration would have been $2500, and I spent $1700.
If you don't want to build it yourself, FALCON NORTHWEST.