If you really want to do some gaming, buying a Mac is not your best option. They do run EXTREMELY hot, they are not specced well for gaming and there's compatibility issues.
If you are talking desktop, I agree that you can get a better performing PC computer for less money, but the OP is talking laptops and MacBookPros are very competitively priced with like-equipped PC laptops. As far as I can tell, it's performance is also competitive. However the caveat is that you would be running Windows on your Mac to get the best gaming performance.
Here is the
PCWorld Apple 15-inch MacBook Pro/2.66GHz (Core i7, Spring 2010) as compared to the
Alienware M17 both are about $2000 U.S. In addition the Mac got 4/5 stars while the Alienware got 2.5/5 stars.
In the
Top 10 Desktop Replacement Laptop list the No.1 spot went to the
Dell Precision M6500 which sells for $4k+.
For the OP if by chance you are afraid the Mac reviews at Mac centric sites are bias, go to PCWorld, select laptops, select Apple and review what PCWorld thinks of them. Here is a
PCWorld Apple link.
I'll say it again, if you can only afford or want to drag around 1 laptop, and you have an affinity for the MacOS, but want access to all those games that don't get ported to the Mac, then the MacBookPro wins hands down. But if all you want to do is play games in a Windows environment, then there really would be no reason to buy a Mac of any kind.
I know they exist, lots of people who choose Windows, but I'll bet you that for the vast majority of casual users, that have traditionally been drawn to Windows, it's because the perception of less upfront hardware costs. It's hard to argue that as long as you put the other Windows headaches out of your equation. But as someone who uses both OSs, I can't imagine using Windows as my primary. It's that simple! It's an opinion based on experience. But I don't insist anyone agree with me.
Back to the original subject, I'm not convinced that DX11 adds substantially to the gaming experience. Graphics are important, but it's the other aspects of the game like, genre, concept, playability, scalability, etc, that are the primary drivers. However, I think you'll find that XP is about to run out of it's useful life in which case an upgrade of Windows would be in order.
I am currently using 32bit Vista. Anyone know if Vista will be DX11 compatible? If I was to guess, I'd say no, but...