Well actually guys I will be installing Tiger. I have the CD's that came with my MacBook and have yet to see a need to switch to Leopard.
There isn't a 'need', so much as an 'opportunity'. I wasn't exactly lined up at the Apple store when Leopard was released, but when santa delivered my new SATA 2.5" Toshiba 160GB disk a couple months later, that was the perfect time to go out and get my copy of 10.5 (family pack) and perform a new install on a fresh disk, which would then be the least problem prone method. My old internal drive now lives in a USB enclosure, and can be drafted back as the original drive again, complete with a working booting Tiger if I found anything about Leopard that
* I hated (haven't found anything, and plenty of goodness)
* Broke something critical to how I use my Mac (there were a couple of moments, but so far everything has worked out there. Well, okay I haven't tried putting Parallels back on yet, but I'm probably going to go with VMware Fusion anyway and I still have a separate PC for Windows necessities, which are rare)
* Otherwise FUBAR'd my Macbook, or some aspect of what I do with it (I can do everything I was able to do, under Tiger and more.)
So really, it's more of a logistical kind of thing. You can upgrade to Leopard while you have a fresh disk to do a new install, or you can consider this to be an incremental thing and concentrate on getting the new disk in, and Tiger installed getting you back to where you were with the old disk and be guaranteed that everything YOU use your Mac for will still work and in the way you are accustomed. Perhaps in the future you'll upgrade the disk to one even larger, and at that point it will be Time. This is only my second OS update having started with Panther, and each has been relatively painless compared to certain other OSes (and I'm not necessarily implying Windows; I'm looking at you Fedora Core/Ubuntu/Gnome/KDE/flavor of the month.)