Before you buy anything I would recommend downloading the
Windows 7 upgrade adviser and make sure your machine even supports 64 bit - some computers, especially laptops will not.
You should also look into
Windows 7 Easy Transfer to migrate your accounts & settings to the new OS (if you choose the more stable fresh install)
I suggest you look into the two programs before making your final decisions.
Please NOTE even the W7
Upgrade disks allow for a fresh install, since you own Vista, don't waste the money buying the
Full version of W7.