Myself, I prefer Gentoo for older PC's. It allows you to configure the system for just about any hardware, and it will only install the software and drivers for the hardware you specify. However, it is a good idea to have a full listing of the hardware in your system to do this. It also requires a lot of time the first time you install it. However, it will result in a system that is highly tuned to your needs and environment. Not for the faint of heart, but a great linux learning environment.
Sound advice, woboyle, since it would appear the thread owner is someone well familiar with the technical side of hardware and operating systems given his stated goal to develop an RS-232 interface. I have had good success with Gentoo in these situations.