Q: Can I upgrade my MAC G4 to OS X 10.6?

Sorry, no way possible, the highest any G4 or any PPC based Mac can go is 10.5.8

You need to buy an IntelMac, maybe a used one?

Some older versions of Protools only work on PowerPC Macs, and some work on both Intel and PowerPC Macs.  The last version of Protools to run on Powermacs is 7.1, but 7.0 and 7.1 specifically were designed for G5s which you don't have.   https://en.wikipedia.org/wiki/Pro_Tools

There is a 6.0 version of Protools designed to work with Mac OS X though.

And your G4 can be upgraded to 10.5.8, although you'd lose access to Classic on that partition or drive.  The last Mac OS X version to support Classic is 10.4.11.  You can dual boot 10.5.8 & Mac OS 9 on some PowerMacs:

http://discussions.apple.com/docs/DOC-1757