Yes, 939 is good, but whether you want dual core or not depends on what you use your pc for. If you are intending on running games, like me, I would go for single core. I myself have an AMD 64 3200 Venice, and I've got it OC'd from 2GHz to 2.4GHz, it runs completely stable, with a max temp of about 40c under load.
I still use the stock cooler, but as for not buying OEM, I would say that there still is point in it. Retail comes with a 3 year warranty, but that is only if you actually use the stock cooler, otherwise the warranty is made void (dunno if they can tell if you haven't been using it). As for OEM, it may only come with a 1 year warranty, but it is obviously not made void by using a different cooler. This means that you should be able to push more out of your processor, with a better overclock.
Also on the overclocking front, if you are overclocking your FSB, you would probably need to lower your RAMs clock speed in the first place, as then you can overclock more, and keep your system stable. However, I've never OC'd an Athlon xp before, so I do not know what feature you would need to find in order to change your FSB multiplier (If it exists).
I have seen (with my
OWN eyes) an overclock to 2.7GHz on an AMD 64 3200 Venice, running stable, at a temp of about 35c under load. This was also with air cooling, using a Zalman CNPS7700-CU. If anyone has seen a better overclock I would like to hear.
Anyway sorry for blabbin on - a bit of a newbie to forums you see