It looks like this offer will do a clean install but if it doesn't find and existing - and what thinks is a properly licensed - version of Windows on the machine it will ask for a serial number for the old version.
One potential gotcha is support for 64-bit applications and peripherals - I'm not a gamer so I've got no idea how likely it is but you might want to make sure that your fave games will run on a 64-bit OS and that 64-bit drivers are available for any special gizmos you play with. I think that as a rule of thumb Vista 64-bit drivers will work with Win 7 but I stand to be corrected on that.
Lastly - I don't know if you've got any experience of Win 7 but I think you'll find it much better than Vista!