MacOS X 10.7 is designed to be an upgrade from 10.6.8 to 10.7. It can, though its advanced, be installed as a clean installation, but thats for Pros.
That said, you should be fine with a Time Machine backup
Note that MacOS X 10.7 actually installs a new partition and puts the installation files on that, so that in the future, you can boot holding down Cmd+R and do all kinds of recovery without the need of DVDs or USB sticks.