You're almost definitely stuck.

Funny thing about 533Mhz. It was both the stopping point and the starting point for cellies. The old PPGA (Plastic Pin Grid Array) cellies ramped up to 533, and the new FCPGA (Flip Chip Pin Grid Array) cellies started at 533Mhz. I'm almost certain you have the old style as the 566 also came out when the 533 did. Have you looked at the processor yet? If the core of the processor is up it's new, but if the top surface is flat then it is the old style.

You also may be able to D/L WCPUID and run it, it will tell you a bunch of info about your processor. Some of the differences between the PPGA and the FCPGA are the PPGA's lack of SSE, and the fab process (.25 for the old vs .18 for the new). You may want to go that rout so you don't have to strip down your whole computer.