For one, I don't think you can overclock Sun hardware at all. Heck, it doesn't even have a BIOS; rather, it uses something called OpenFirmware (the best way that I can describe it without knowing the details is that it's sort of like a limited version of a UNIX shell that's designed to do something similar to what a BIOS does, and more).
Also, Sun hardware typically uses some other form of graphics slot (can't remember the acronym right now). It's not AGP, and it's definitely not compatible with AGP either.
Until our Sun experts that have actually used Sun hardware get in here (I've just read about them, so ignore what I'm saying
), I'd wonder what kind of monitor it is that you're using on the system. Is it one that came with it, or is it a CRT that is attached with an adapter? I'm thinking it might be an issue of the Sun needing a slightly different (and incompatible) type of monitor, or perhaps it's a case where the system was set up as a server, where it just sent boot messages to the serial port, neglecting the monitor entirely.
Edit: BTW, just noticed that these are your very first posts. Welcome to the AT Forums.