I used to run a Duron (right up until a friend managed to fry my Socket A board).
I didn't really notice any problems with the Duron; I even put it to the test with some heavy-duty 3D games such as Descent3, and it performed quite nicely (and with only PC100 Ram and an outdated video card, to boot).
I would recommend an Athlon, but a Duron is a nice cheap alternative that also overclocks quite nicely.