It depends on which model of Voodoo card you have - the CPU is irrelevant - it's the same whether you've got a 25mhz 030 or a 66mhz 060 + 240mhz PPC.
If you leave it empty, it will automaticaly select the default for your card - for a Voodoo 3 3000 I think it's 166Mhz but I had mine overclocked to 200Mhz without any problems. There is more information on this
here.
