Well I have a TNT2 Ultra. I'm not sure how much better it is than the m64, but I overclocked mine from 183mhz memory; 150mhz core. Now its at 212mhz memory; 162mhz core. I think yours be won't able to do go that high.
You can always just increase the mhz by 1 and then do some graphic intensive gaming for a while and if it works then move up another 1mhz. If it gives you artifacts or odd colored blocks in your games then set the mhz back to the last stable setting. This can be dangerous though, especially if there is no cooling on it. I have a small heatsink and fan that came with mine.