So you want to run Windows, eh? Welcome to the fold, my friend. The only real question now is, which version of Windows should you run?
Sadly, this is one area in which Windows really is more complex than Mac OS X. Microsoft offers its latest client OS, Windows 7, in a number of product editions, but the reality is you'll likely only find yourself choosing from between the two mainstream versions: Windows 7 Home Premium and Professional. That said, you may actually want to consider Windows XP in rare instances as well. It's not as easily obtained as Windows 7, but it has lower hardware requirements and is thus a better choice if all you're looking for is Windows application compatibility. Put simply, XP will run only marginally faster than Windows 7 on your existing Mac, it will run much more quickly in a virtualized environment. (I will be examining Mac OS X virtualization solutions in an upcoming article in this series.)
So, how do you choose?
If your switch involves buying a new PC and not reusing your existing Mac, get Windows 7. While it's still possible to get Windows XP under special circumstances on new hardware, or on special low-end computers, it's not worth it. Windows 7 offers better security and a much more efficient and attractive user interface than the one provided with XP, and it will be supported with new functional updates for far longer than will XP.
Tip: Windows 7 requires at 1 GB of RAM to run effectively. I recommend at least 2 GB of RAM, however. Memory is inexpensive, so this is a no-brainer.
If your only goal is to run one or more Windows applications, and you're not concerned at all about any Windows 7-specific features, then Windows XP is the way to go. This is especially true if you're going to use a virtualization solution like VMWare Fusion or Parallels Desktop (with which you will run Windows in a virtualized environment).
Tip: 2 GB of RAM should be fine for Boot Camp, with both Mac OS X Snow Leopard and Windows 7.
Tip: For virtualization, upgrade your Mac's memory as high as it will go (typically 4 or 8 GB). Virtualization solutions are memory and processor intensive, and will run better with more memory.
If you do choose to run Windows 7, you'll need to pick between the two mainstream product editions mentioned previously. I explain the differences between the Windows 7 product editions in Windows 7 Product Editions: A Comparison and in Part 2 of my Windows 7 review. To pick an appropriate version, scan the tables in that first article and figure out which features you absolutely can't live without. Then, pick the lowest-end Windows 7 product edition that includes those features. (Lower-end product editions are less expensive.) Generally speaking, if you are going to run Windows 7 in Boot Camp, you will want Windows 7 Home Premium or higher.
Tip: You cannot use Windows' hardware-accelerated Aero user interface in a virtualized environment, so don't use that feature as part of your decision making process if you will be running Windows 7 under Fusion or Parallels Desktop.
Don't be thrown off by the retail prices of Windows. You don't actually have to walk into a local Best Buy or other electronics retailer and purchase a retail version of the OS. Instead, you might consider a so-called "OEM" or "system builder" version of Windows 7, which can typically be had for quite a bit less. There are three differences between an OEM version of Windows 7 (or XP) and a retail version, one of which is major: The retail version comes with a pretty box. It also includes a modicum of official Microsoft support. Most important, though, it can only be installed on a single PC, ever: once you activiate it on a PC, it's locked to that PC. If none of these issues are a concern, then save some money and skip out on the retail version. OEM versions of Windows can be had from a number of online retailers, but one I use, trust, and recommend is NewEgg.com.
So let's recap. You're either going to run Windows on your existing Mac or on a new PC. If it's a new PC, get Windows 7.
If it's on your existing Mac, you will be running Windows in a virtualized environment under Mac OS X or in a dual-boot scenario with Boot Camp. If you will be using virtualization, choose Windows XP. For Boot Camp, either is a good solution, but if you need Windows 7-specific functionality or want to experiment with Microsoft's latest OS, then choose Windows 7.
If you do choose Windows 7, choose the lowest-end version you absolutely need, if only to save money. This will likely be Windows 7 Home Premium.
With XP or Windows 7, consider an OEM version to save even more money.
And if you are using a virtualized environment on your Mac, upgrade the computer to 4 GB of RAM or the maximum amount.