please dont buy a cyrix. i've had experience with two fo their chips, and what miserable experience it was.....
please, please, please.....just get a new barebones system (150-200) with an amd or intel chip. doesnt even really matter if you wanna get a celeron (though durons offer better perf. at a lower price).
the biggest mistake you can make in buying computer parts (assuming your not an enthusiast who regularly forks over inordinate amoutns of cash for the latest and greatest) is to buy equipment to suit your needs for today. buy something that will minimally run future software as well so that you are not constantly cursing your system for being slow, incompatible, and incapable. nothing in the computer market stays current for very long, but if you buy a c3 600, which has been obsolete for a while, your buying WAY behind the curve. durons or socket a athlon tbirds will give you the best price/performance ratio i can imagine right now (maybe even an xp 1600+ since they are ultra cheap right now).
anyway, to summarize: spend a lil more and do a major system upgrade so that your dad can truly function even with tomorrow's apps..who knows, he may even try playing mp3s and whatnot 🙂