I'm doing that right now. Moving from a server running 7.5.4 and RHEL3 to another one running 8.0.1 running RHEL4.
Best way is to use the included migration tool. You just use the migration tool on the new server, wait for a while and then it's done. Or, it's supposed to be that way. I had some minor problem because I had forgot to configure mailman and postgresql on the new server. But I configured these, deleted all the clients on the new server and migrated again. Works like a charm now.