Go straight to version 8. It's such a step beyond ver 7 in terms of feature development, solidity on Leopard/Tiger and interface changes, that it's pointless to take an intermediate step. However, bear in mind, that as per usual, any version of QuarkXpress will only let you save down or export files to the previous version.
I wouldn't advise making a clear switch to InDesign unless you're free from having to use legacy templates and dealing with others who are also using Quark. InDesign takes a little getting used to and you may find yourself frustrated, especially if you're a keyboard shortcut whizz with Quark.