You could alway just but a PCI modem and disable the onboard one! May be easier that going through all the hassle of getting a new one out of the manufactures.

I dont know about in America but you can get a pretty good one for £15 in the UK!

And you have built a machine before so it should only be a two minute job! And if it does'nt improve you connection speed you can aqlways take it back the same day and say you dont want it!!