It sounds like this machine already has the maximum RAM, ie. 64MB. n overclock from 75 to 80MHz would make barely any noticable difference and would be more trouble than it's worth. If you wish to squeeze more power out of this machine I would suggest a motherboard from a later 6xxx series, as already suggested.

Despite the popularity of the 6200 in the past, it did have quite a few technical problems largely surrounding the fact that th motherboard was basically a re-used 630 motherboard. This meant the processor couldn't perform to it's full potential, effectively cutting it's performance in half. This issue was eventually resolved in the 6360.