8. It's not really necessary if you have 7, but it's super necessary if you don't
As a gamer, you don't need to be in a hurry to upgrade to Windows 8 if you're already running Windows 7. The new interface is clearly intended for small touchscreens, not high-res monitors. Windows 8 runs slightly faster than Windows 7 on the same hardware, it boots up faster, and there are nice tweaks to Windows Explorer, file copying, and a few other aspects of the desktop. But the differences between 7 and 8 aren't large enough or meaningful enough that gamers should feel compelled to plop down good money for an update.
If you're running Windows XP or Vista, though, Windows 8 is a huge step up and you should probably upgrade. Microsoft really wants you to do this, so they've priced 8 to move. $40 gets you an upgrade from XP, Vista, or 7. That's the cheapest Windows upgrade ever. If your poor PC is still running Windows XP or Vista, do it a favor and update to a faster, more secure, more modern OS. If you're coming from XP, the jump from DirectX 9 to DirectX 11 alone is worth the price. Oh, also. The cheap upgrades expire January 31st, 2013. After that, prices go up.