We converted our 2006 MacBook Core 2 Duo dual USB to Lion a few weeks ago.
Don't bother; the MacBook isn't up to it. It's too slow.
I suspect more memory or an SSD drive would help, but, really, six years is a long time for a laptop.
I think we're due for a new machine. When we get one I'll wipe this machine and revert it to Snow Leopard. That would mean no more iCloud, which is, at this point, a feature.
Incidentally, it has the flaking plastic border problem that's common with older plastic MacBooks. It started doing it four years ago and has been stable since; we use scotch tape to close the gaps. A friend tells me Apple will replace the broken plastic for free, even on quite old devices. The hassle isn't worth it for us, but it would be nice if that were true.