Replacing OS X with Windows/Linux on aging Mac
Do you have the original install disks for Mac OS X? If so, there shouldn't be any issues with wiping the Mac partition completely off the hdd. 

However, if you don't have those disks, I would strongly recommend keeping the Mac OS X install and creating a new partition (shrinking the Mac partition) for the new OS. Mainly because if something goes wrong (or if you don't like either alternative) it's much easier to recover a MacBook using the Mac OS. I think you can get away with 5-10GB for the Snow Leopard partition. 

If you have the original install disks, you shouldn't have an issue with restoring it. 

Linux will take some tinkering to get working properly. I had a particularly rough time getting wifi to work in Ubuntu 12.blah on an older MacBook Pro. Plus side: With a laptop that old, there should be a decent amount of documentation for making your hardware work with Linux. I'd stick with Ubuntu if you like fancy, or Linux Mint if you like simple. Windows 7 (if installed through Bootcamp) works pretty well without much hassle.