You show 1,9Ghz because speedstep/C1E is active, that is normal and doesn't affect performance when needed. See my 3DMark pic above, it also shows only 2,5Ghz in idle, while being 3,2 under load.

Where do you start? Easy, go into bios and set FSB to 400
Not much options there to mess around with anyways...
You do have DDR2-800 FBdimms, don't you? Make sure you have a fan over them, as they are gonna get hot when running 400Mhz+
No need to raise any voltages yet. Just try FSB 400 straight, it should work. Use Prime95 to test stability, and monitor your temps closely. You can use Everest for CPU temps, substract 10C from the reading though as it uses the wrong Tjunction just like coretemp.