The memory and the OS is quite straight forward but the HD is a little more tricky.
It requires minor, or depending on your level of skill, major, surgery to get at the drive. There are a number of good sites around with step by step instructions on how to do it.
Yes, it makes sense to replace the drive and install 10.5 onto it. The old drive can be used in an external enclosure but be aware that the drive is SATA.
Apple will do it all for you, probably at a price that is close to the cost of a new one. It would not have been done under AppleCare or the original warranty.
It's a nice weekend project
I've got an older Mac Book Pro (17") and I'd like to upgrade almost all of it:
Memory: 2GB -> 4GB
Disk: 160 GB -> 320 GB (or better)
OS: 10.4 (Tiger) -> 10.5 (Leopard, or Snow Leopard)
Since I am upgrading the disk does it make the most sense to just replace the disk and install the new OS? Can I do this myself or do I need to take it to an Apple Store? I know this voids the warranty, but that's long expired, and so has my one year of Apple Care. Still, is it better for Apple to do it? The same question for the memory, too.
Also, I assume I can take the old disk and put it into an enclosure to turn it into an external drive.