Yes, you should not touch the voltage.
With stock cooler, stock voltage, a CPU can easy be overclocked 20-25%. In this case 3.4Ghz should go without problems, and it will be stable.
But be sure that you monitor the temp. of the CPU. If it will go over 60C, set back the freq.
And the X1600, or X2600 (it should be with GDDR3) would be a good choice
:)