XboxOverclock will allow you to overclock the FSB and NVCLK via software. It uses infromation gained from XBOverclock by WulfyStylez.
See the requirements for nxdk.
git clone https://github.com/GXTX/XboxOverclock
cd XboxOverclock
eval $(../nxdk/bin/activate -s) make -f Makefile.nxdkThen you'll have the xbe in the bin folder.
Once you boot into the xbe you'll see a prompt. Use the left and right D-Pad to change the wanted FSB frequency, use the up and down D-Pad to change NVCLK frequency.
Both over, and underclocking mess with the RAM frequency; the RAM on the Xbox is already overclocked out of the box thus the max it'll run is around ~235MHz, so to get higher CPU clocks you'll need to raise the memory divider. On a stock, recapped, 1.0 system I am able to run ~1077MHz (not exactly stable but running). Perhaps with some VCORE mods you'd be able to push this slightly higher, but I imagine a 40% OC is already pushing what this chip can do.