My wife has an old and busted Curve, so I got her a Tour for her birthday next week. It currently has 4.7.1.61 on it, and I'm sure it could be upgraded.
I would recommend 5.0.0.419, either that base, or with a .419 based hybrid. I can't post links yet, but if you look into the tutorials section you should find an easy explanation of how to do it. There are newer OSes available but .419 or .484 are generally accepted to be the most solid available, besides hybrids.
