Operating System: Windows XP or Windows Vista (or both!)
Unfortunately, there are no simple answers at the moment when it comes to choosing the right operating system. There are really two main questions to answer:

1. XP or Vista?
This question used to be an easier decision, since there were a fair number of applications and drivers which had serious trouble under Vista, but 95% of those issues have been solved. Games still tend to run faster under XP, which makes the venerable OS attractive for a gaming rig. On the other hand, DirectX 10 (microsoft's rendering technology which new games are being based on) will never be released for XP, so there's going to come a day when new games just won't work with it. This issue is closely tied to the next question, too:

2. 32-bit or 64-bit?
There was supposed to be this major breakthrough when 64-bit processors finally teamed up with 64-bit applications, and everything was going to work much, much faster. Only it never happened. We've had both options for years now, and not only are most applications still being written in 32-bit versions, but even the 64-bit ones don't really perform much better than the 32-bit variety. It's been something of a disappointment to say the least.

Nevertheless, 64-bit Windows (which can run 32-bit games just as well as 32-bit Windows, by the way) does bring one potentially valuable thing to the table - the ability to use more memory. For reasons which we won't get into because your eyes will bleed, 32-bit Windows can only make use of between 2.25GB and 2.75GB of memory. Any additional memory you install in a 32-bit PC just sits there and looks pretty. At the moment, 2GB of memory will probably get you by just fine for the next couple of years, but there are already a few games out there which do perform better under 4GB. (It's going to be a while before any games need or even use more than that.)

Here's a chart to help you visualize the options:

XP 32-bit	Solid and proven. Best gaming performance, but no DirectX 10.	Highly Recommended
XP 64-bit	Potentially troublesome driver issues.	Avoid
Vista 32-bit	Solid, but limited to less than 3GB of memory.	Meh.
Vista 64-bit	The way of the future. You can't avoid it forever.	Highly Recommended

As you can see, we tend to rule out XP 64-bit right off the bat. It's kind of the red-headed stepchild of the Windows lineup, having been neglected by a lot of hardware developers. It's a solid OS, but you might have trouble getting drivers to work for newer hardware.

We're not too interested in Vista 32-bit around here, either. It's just as stable as XP 32-bit, but in our opinion, if you're ready to make the jump to Vista, then you're ready to go 64-bit. Otherwise, you're signing up for a crippled version of the new OS, and you'll have to upgrade later when you want to use 4GB of memory in a year or two. So here is what we recommend, depending on your situation:

Your primary concern is gaming performance, period. You'd be willing to stick a tinfoil antenna on the top of your PC if it meant you'd get 3 more frames per second in Crysis: Get XP 32-Bit.

You want to experience the higher-quality visuals DirectX 10 has to offer, and you want to be prepared for games that really need 4GB of memory to shine: Get Vista 64-bit.

You have to have it all. Seriously, it's all out there and it's all yours. And you want it. All of it: go dual boot XP-32bit and Vista 64-bit.

One Last Thing: What Version Of Vista?
The choices are: Home Premium, Business, and Ultimate. From a stability and game performance standpoint, there isn't any difference, so it really boils down to features:

Vista Home Premium	Media Center DVD Maker HD Movie Maker	$110
Vista Business	Faxing Remote PC access	$140
Vista Ultimate	All of the above "Ultimate Extras"	$180

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