That said, it's always better to do a fresh install than to upgrade. You get that new computer smell.
If you've got the time and inclination to set everything up again, I say go that way. However, if you do decide to take the easy route and upgrade, you shouldn't have any issues.
The most problems I've had with Leopard involved updating from previous versions. When 10.5.0 was announced, there were only a handful of machines that upgraded without incident from 10.4.x (in my experience). Anything before that, 10.3/.2, was a surefire way to cause whacky problems with a system.
10.5.1 had some better results, but still had some weird issues when upgrading. 10.5.4 (or is it .6 at retail now?) may be better, but I would suggest performing an Archive and Install while preserving user data and settings at the very least, if not performing a backup and clean install and migrating data over.
All of the problems I've seen weren't permanent and were never too serious, but typically ended up in a fresh install/archive having to be performed anyway. I never have lost any data during these issues, but there is a very slim possibility of that happening (for any update on any system, Win/Mac/*nix).
I would suggest performing a backup/clone with Disk Utility or one of the free third-party apps like Super Duper.
It's always better to be safe than sorry, especially when so many things were updated and changed in 10.5.
Also, Will i lose any applications/programs that have been installed and any other information?
I guess its not a huge loss, but would defiantly be nice to keep it all.
If you do an Archive and Install while preserving user data you won't, nor will you if you make a clone and perform a clean install and then use the migration assistant afterward.
Some things can't be copied over in these situations which typically include third-party system applications like ones that add a pane into the System Preferences, and third-party printer drivers.
By the way, when installing Leopard, I suggest hitting the Customize button which appears just before the installation. You can deselect all the printer drivers you will never use, and deselect all the language additions and extra fonts if you don't plan on using your version in a nonstandard language; saving you several gig's of space.
