Best 56k modem for OCing

I heard that it is best to go with an external dial up modem that goes into the serial port. But, which is the best for OCing?

i have heard us robotics externals were best of the best.. although you pay for it $$$.. i wouldnt think it would matter, if it was external.. i mean you're going through either the usb or the serial port.. high fsb speeds usually dont effect them too much

So you're saying that it doesn't matter if I go internal or not?