Perhaps not that much help but I moved a 4.2 site from 8.1.7 to 9 without too much trouble and I'm running 5.1 on 10g now. All I did was load the data and fix what didn't compile. Most of this is because of keyword changes and package definition errors. There were not that many things and I suspect an older site would be easier. The other thing to watch out for is the performance is different. Most things are faster but a few are slower and I did not find all the slower ones until it was in production. The only way to find these is load test.
