Re-instaling the OS I'm fine with, once everything is assemble, I'm pretty much good to go I think. I've just never actually assembled a computer. As for my tower, it's fairly large, so it should be able to handle any size.
Then I would say, buy yourself a motherboard.
Check if the new memory stick you have fits, if not you will need additional memory,
check if you can re-use your CPU, if not you need a new CPU,
check if you can re-use your graphics card, if not you need a new one.
Make sure you can connect you Harddisk (if its 'old' IDE (now also called PATA) make sure you have a motherboard that can connect at least enough of those)
make sure you can connect your CD/DVD/whatever you have.
Backup your data to some external device, or another computer.
Start by picking up the screw-driver