Like they said ^^ ...the K7S5A, at least with the stock BIOS, is just not the right tool for the job if you want to OC. To OC, you need somewhere to OC to. The AthlonXPs use at least a 266MHz bus right out of the box, and that's the fastest that the K7S5A supports... no jumping to a 333MHz or 400MHz bus is possible, and I don't think the PCI is clock-locked on SiS735, so trying for a 150MHz (DDR300) bus speed could result in Bad Things happening to your hard drive :Q
For a budget OC board, get a Shuttle AN35N Ultra. Or simply buy a 2400+ for your K7S5A Pro and call it good