Does CPU-Z show the PM 965 chipset or is it the GM965 chipset with integrated graphics?
The chipset supports the 800 MHz bus speed but if the bios does not recognize your CPU, it will probably default to 667 MHz instead.
You need EIST to be enabled to adjust the multiplier and voltage. The bios might disable EIST when it has a CPU that it doesn't know what to do with it which can leave the CPU locked at the minimum multiplier and VID.
When there is a problem with the power adapter, some Dell laptops use clock modulation throttling as well as the minimum multiplier which really kills performance. Hopefully ThrottleStop can help out.