FCP 6 to 7 is pretty smooth. It can get a little tricky when you're dealing with clips that FCP generates (like the Text Generator may format text differently so you have to go through and tweak each title card). Also, if you were using LiveType files in a Sequence, those clips may render differently as well.
If you're not doing it already, make a duplicate of your FCP6 project file in the Finder. Add something like "_FCP6" to the original project file and "_FCP7" to the duplicate, opening the duplicate in FCP7 to update it (it will permanantly become a FCP7 file when you save). Alternatively, you can open the original FCP6 file and use File > Save As to save a FCP7 project.
Also, as long as you're running a version of Mac OS X and hardware that supports both FCP6 and FCP7, you can have FCP6 (Final Cut Studio 2) and FCP7 (Final Cut Studio 3) installed at the same time. Before you install Final Cut Studio 3, go through your Applications folder and add the version number to each of the Final Cut Studio applications (Final Cut Pro should be renamed "Final Cut Pro 6", DVD Studio Pro should be renamed "DVD Stuio Pro 4.2", Motion should be renamed "Motion 3", Compressor should be renamed "Compressor 3", Soundtrack Pro should be renamed "Soundtrack Pro 2" and Color should be renamed "Color 1"). If you don't, the Final Cut Studio 3 install will replace the Final Cut Pro Studio 2 applications with the same name (if I remember correctly Soundtrack changed names slightly, but I would rename it anyway) Of course, double-clicking a FCP file will use the newer version, so you may need to use drag and drop or control click > open with... to open your FCP6 projects in FCP6.
If you'd rather not run FCP6 and FCP7 on the same startup volume, you can clone your boot drive and then install Final Cut Studio 3. You can reboot for the version you want to edit with (as Michael suggested in his post).
-Warren
