You won't get a huge increase overclocking an Athlon 6400. Athlon x2s don't really overclock that well, and I only ever really put my 2.6GHz 5200 to 2.8GHz and it ran fine.
All you do to overclock is basically up the bus speed. It'll be at 200MHz default, and with the 16x multiplier your CPU has, 200x16=3200. You might be able to just up the bus speed to 213MHz and that will give you 3.4GHz, without having to worry about voltages.
If you want to go higher than that, then you're going to have to lower the FSB (do this by dropping the multiplier from 16 to 15) to keep it within 900 to 1100MHz (safe zone) and start upping the voltages, and also underclocking your RAM, as when you raise the bus speed, the RAM speed gets upped as well with the FSB. Though to work out how much the RAM has been overclocked, you just take the amount you upped the bus speed by, so say you put it to 213MHz, you increased by 13MHz and you just double this number and add it on to your RAM, so at 3.4GHz, you RAM will be at 826MHz (I think, I could be wrong).