Welcome to the Apple discussions.
You know you'll probably spend more time bringing your PB up to 10.5.8 than replacing your MBP hard drive. It's not difficult to convert, but there are certain tasks you need to follow. Read through this installation note to be aware of potential issues:
http://docs.info.apple.com/article.html?artnum=307005First, assume you have a retail set of 10.5 Leopard disks, as you can't use a set from, say, an MBP, which are locked to the system type they were delivered with. If not, call 800-MY-APPLE and order a set, assuming they still have it in stock.
Next, you need to look your existing software over to see if anything non-Apple has been updated to work with Leopard. One reason people have issues with the update is they'll have drivers or non-Apple software that hasn't been updated to work with Leopard. When you bring up System Preferences, is there any software listed under 'other'? When you look at login items, is there any non-Apple software listed there? Those would be two sources to look at. Do you have any non-Apple printer/scanner drivers installed? Look down your applications list too.
Once you've got necessary software updated, then you can do the upgrade. Install whatever version of 10.5 you have, then the easiest thing to do is to run Software Update several times to get needed Apple updates done. Personally, for operating system upgrades, I prefer to run the
combo updates from the Apple site, then run Software Update. If you choose to do this (there may be two for Leopard, depending on the base level installed), repair permissions before and after installing the combo update(s).