Can we have some details of your system, please.
At the moment, if you have sufficient space on your HDD, I recommend a dual (or multi) boot system. This is because W7 is still in Beta, and will cease working in a few months time, meaning that if you just had W7 on your system you would be left with an inoperable machine when this version of W7 expires. At least, with a multi boot, you can still have access to any documents you have created in W7 from Vista.
I would still recommend this for the forthcoming RC release of W7, and would only consider installing W7 as a sole OS once it has been officially released to retail. Even then, I would still have a multi boot system - after all, I have paid for Vista and it is still supported, plus setting up a multi boot system these days is a doddle as everything is automatically taken care of.