It depends what kind of motherboard and processor you got. I have a Celeron 400mhz and a Tekram i440BX-A motherboard, all I did was change a jumper setting on the processor and then in the system BIOS I changed a frequency setting (can't remember the exact switch).
As far as the GeForce2 MX400, I recommend a program called NVMax for that. I have my core/memory set to 240/350.
It's all trial and error, just make sure you have good cooling (or a good house fan, which is what I use).
Forgot to add, I have my Celeron set at 500mhz now, but you got to make sure you have adquate cooling.