If you do an Archive and Install there is an option to preserve your users and preferences, and other stuff. So you get Leopard with all your old settings, and a file of the old stuff that you can delete if everything is hunky dory.
If you have an external hard drive you can clone your existing Tiger over to it using the free Carbon Copy Cloner or the free trial version of Super Duper (or you can do it by coping over the correct stuff using Terminal and blessing it bootable; if you find the description of how to do it). Then you can do a clean install on the internal hard drive that will erase everything, but at the end it gives you the option of importing the settings, preferences, etc. from the external bootable. You can do a similar thing with a partition on your internal HD, but if you don't already have a partition it will erase everything to divide into two.
I bought an external in preparation for using with Time Machine in Leopard. I partitioned it into two, and cloned Tiger over to it using the trial of Super Duper. Then Initially I did a simple upgrade to the Tiger on my internal. To make a long story of re-installs, bug fixes, crashes, softward incompatabilities short; I now have three partitions on my external: one for clone of Tiger, one for clone of Leopard, one for backup. I have two on my internal: one for Tiger and one for Leopard. The Leopard version I'm working from is a clean install, porting over preferences, etc. from Tiger on the internal partition. Some things don't work in Leopard, so I can boot from that if I need to. Nice thing about having clones of both on the external is that you can apply upgrades and stuff to the working versions on the internal, and if something gets screwed up you can clone back over from the external and get right back to where you were. Once Leopard is working well enough, I will likely remove the Tiger partition from the internal and just have Leopard on the internal.
A couple of things to keep in mind: at the present time Super Duper is not compatable with Leopard, only with Tiger. Carbon Copy Cloner works for both. You can change and create partitions dynamically with Leopard (without completely erasing the drive) - this is one of the best features of Leopard, in my opinion.
