It's been awhile, but it looks as if Microsoft has released an OS they can be proud of. If you are still running XP I would say upgrade, as long as you have enough memory. A minimum of 2GB for Windows 7 to run well, more RAM is better. If you're running Vista, absolutely upgrade.
The next consideration is which version to get. Home Premium, Pro or Ultimate and 32bit or 64bit?
I would go with Home Premium. You should take a look at the side by side comparisons of the flavors to see if Pro or Ultimate have something you absolutely need, but Home Premium is very feature rich.
On the 32bit vs 64bit front I would recommend 64bit for everyone with a 64bit capable processor (CPUs made in the last 3 years). There are some remaining driver and program incompatibilities, but most have been fixed. Go through your list of the hardware you run and make sure there are 64 bit drivers available for your components from the manufacturer. Microsoft has a Windows 7 Upgrade Advisor that you can run that will tell you if there are any potential driver and software incompatibilities.
Buy the Full OEM Version of Windows 7 Home Premium 64 bit at Newegg.com $109.99
This is the OEM version which means you won't receive any official, phone, chat or email support from Microsoft. If you think you need the support, you can get the Retail Box full version here for $199.99, or if you have Windows Vista you can get the Retail Box upgrade version here for $119.99.
Since you're reading this you have either already built your own PC, or are about to build a computer, so you probably don't need support from Microsoft.