I'd still put the modem on the silversurfer though. Like others have said, the lower CPU overhead means more CPU cycles for web browsing etc. So although the transfer rate won't be any quicker, pages might display sooner.
40-odd kbs dialup isn't bad, I've seen a lot worse on modern PCs. There could be many things stopping you getting the theoretical maximimum.. noise on the phone line, phone extension cables run near noisy equipment, crappy modem rack at the ISP etc.
I wouldn't bother with a serial port mouse, but go for an adapter that goes in the usual mouse/joy port, they don't need drivers for basic operation (AFAIK, drivers are only needed for scrollwheels and extra buttons) so they'll work with games that don't use the OS.
