They don't even make 500MHz processors anymore. LOL Any modern computer should blow that requirement away. As previously stated by others, the "MHz" rating determines the speed at which the Processor (CPU) cycles. The higher the number the better.
However, as simon12 seemed to suggest it's not all about MHz. It's like cars. Two vehicles with the same Horsepower don't travel from 0-60mph at the same rate. There's a lot more under the hood, in the drive train, etc.