I think you are confused - There is no such thing as a Pentium III running at 433MHz. The slowest P3 would be a Katmai Slot 1 P-III at 450Mhz. Do you mean 533Mhz @ 133Mhz FSB, or are you talking about a 433Mhz Celeron, running on 66Mhz FSB? They are two very different chips and would directly impact whether or not you can use them in the board you have.
Give us the specific model of motherboard you have. I will tell you though, at the absolute most, assuming you have a board that fully supported 133MHz FSB, you would get a 1400Mhz P3 using the extremely slow SDR Ram.
The cost of the adapter alone for a 1GHz+ Celeron or P3 would probably not be worth it, with the cost of a replacement desktop so low. If you are on an extreme budget, you could recycle the hard drive, optical drive, chassis and potentially power supply in a replacement desktop.