I am assuming you have the Celeron 1.4Ghz chip in your M320.

When upgrading a CPU on an OEM system, often times the motherboard is going to be the limiting factor. For that reason I always try to find a chip that Has the same socket (Obviously) Runs on the same FSB, and will run on the same chipset.

Your Celeron M that comes with the M320 is socket 478 and runs on a 400Mhz FSB.
That is the same Socket and FSB as the Pentium M that was also offered in the M320.

They are fairly cheap on Ebay. But I have only been able to find a 1.5 Pentium M. No 1.7 or 2.0. An example auction can be found by clicking HERE

Now all of this is several generations ago, and while the Pentium M is a faster chip then any Celeron of the same flavor, don't expect it to be competitive with things on the market today.

If you have anymore questions just ask.