You could keep all the packages for your vid/sound drivers if you needed them (i.e NVIDIA-Linux-x86-1.0-5336-pkg1.run, etc.), and any user-based configurations is stored in your home dir, so you could switch out your whole distro and have the same wallpaper, icons, click behavior, etc. Basically anything you're storing in /home
Mandrake makes creating partitions easy in the install (and can supposedly even do so non-destructively to an extent, but I've never ventured to try it). You just click "Custom" or "Advanced", whatever, and define the partitions manually (iirc you should be able to even "Base Partition Setup on this Scheme" and have it auto-assign /, swap, etc. and then resize / down and create a /home partition). Anyhow it's simple to do and has lasting benefits down the road. I've gone from Red Hat to SuSE to Mandrake, all the while keeping my data stored in /home. You may have some anomalies due to differences in distros, but nothing too terribly noticeable, especially if you're just doing an upgrade.