Kudos for approaching this task If I had a second N900 I would be trying this right now
Up-to-date libraries (inc. libc)
Up-to-date stable OS
Up-to-date stable packages with a debian package manager

Up-to-date kernel and this would make the device more of a joy

Right now I'm on CSSU-thumb and it's great (specially for the old kernel + libs used) but an overall upgrade would be great for porting more up-to-date applications/kernel modules without too much of an effort (specially because the debian folks do some of it for us)

This would be a great way to keep the system updating, depend on debian

Anyway, I hope I have put my enthusiasm forward enough and just wanted to thank you more actively than just clicking a link