i built computers, then i went mac 3 years ago. I love OS X.
If I was going to buy a computer now, I would buy a pre-built win 7 box. 6 reasons:
1) windows 7 is great. Its faster, easier, and cleaner than vista is. It can run on much weaker hardware (netbooks, nettops), and much better on good hardware.
2) PCs have gotten so ridiculously cheap compared to macs, they are now at least 50% less expensive than macs. Go price out current desktops/laptops. You can get better stuff in a pc for half of whats in a mac. As much as I love OS X, most people probably don't get the same benefits out of it that I do, since they don't need unix commands. I just helped my girlfriend buy a laptop and at the end of the day, as much as I wanted her to have a mac, there was just no way to justify the extra cost.
3) if you get the right vendor you will have a single point of support, instead of one for each part in your build
4) it used to be you had to build in order to a good priced box for gaming, thats not true any more. If you are really into building computers (that is you enjoy it) thats one thing, but if you are just doing it to get costs down, its not nearly as beneficial in that respect as it used to be.
5) Yes you can run windows on a mac but to do so legally means you are going to shell out an extra $100+ for the win license. Without windows, you can't run a lot of apps that seem to take forever to get ported to macs (like google chrome for example). Sometimes its nice to just be able to natively run the apps you want without dual booting, or virtualization and without extra licensing fees
6) the cost to vendors for those windows 7 volume licenses is much less than what you will pay, so if you are going to buy windows 7 for your build, you are already at a disadvantage.
Macs need to get cheaper, you can get good pc laptops for $500-$600 now, and good desktops for the same. An $1100 macbook or $2000 macbook pro doesn't have the value proposition it used to.