short answer: YES
long answer:
just because you can... doesn't mean to should.
Although its easier to upgrade than do a clean install. A fresh start is usually the best idea. You can always save all your game data to an external drive, and then retrieve it later. When you upgrade, you are sometimes leaving extraneous pieces of the old OS that take up space and system resources. For someone who games and is serious about their PC its always better to do a fresh install. Even if you have the 'upgrade' disk, and not the Retail disk, you can do a fresh install.
BTW, i'd wait a month and get Windows 7
not saying Vista is all that bad... i use it
but Windows 7 will be much easier and nicer to use