As ickymay said, unfortunately you won't be able to simply "roll back" to the vista installation... So a dual boot setup would be the best way to go about it.. It's not that hard to setup.. you simply create a new partition (which can be done in Disk Management, which is found in control panel under Administrative Tasks) then do a clean install on the new partition.. After you've installed Win 7 on the new partition you'll then be presented with a choice of which OS you'd like to boot into when you start up your PC..
I say go for it... you won't regret installing Windows 7 in my opinion.. there are tons of people having nothing but success with it and loving it... myself included.. I can't wait for 7 to be done and in stores so I can go buy a copy..
And yes I'd suggest backing all your important files up before you do any of this.. that's always a good idea (you never know what COULD happen when installing a new OS, though if your running fairly modern hardware you shouldn't have any problems).. That way too if you do install 7 and for some reason don't like it then you can just wipe out the new partition and go back to your original setup of Windows Vista... but like I said, I doubt you'll dislike Windows 7..
If you run into any problems, feel free to ask for help, we're always glad to help out..