Many of the concepts learned on the PC can be transfered to the Amiga. If you are going to code demos for OS4 you can certainly use C. The PPC processor has plenty of power and learning assembly for it (although interesting and possibly helpful) is not needed.
OpenGL is essentially the same on all platforms. I've used OpenGL on the PC and on the Amiga and the only difference is in the OS specific stuff like window setup, event processing, etc. The actual OpenGL code is portable.
So if you want to learn to code on the PC the knowledge will apply to the Amiga also.
