I would think that would be fine. My 3000+ is slightly overclocked and the stock heatsink never even gets warm. I Would try it first, then if the temps are high or you want to overclock invest in a new heatsink. My thought is don't spend the money unless you have to.
Uh- guys, hold on a second.....If we are talking about stock hardware I would in no way reccomend to someone to use the same heatsink/fan. The stock heatsink and fan that comes with the X2 4200 is about 50% bigger than one on the 3200+. Buy a retail cpu, and use the heatsink and fan that comes with it, that is my advice.
I have an newcastle (130nm) 3500 and the heatsink that came with it is the same one that came with x2 3800 (well almost the same).
My opteron 165 came with a bigger heatsink with heatpipes. Those they ship with opterons only. I dont know about higher athlons like 5200+.
Your best bet is to check your temperatures. Use a program like everest or speedfan. Even the software that came with your mobo would be good.
And keep your airflow in the case good.
If we are talking about 65nm x2 4200 then I wouldnt wary about. Only if it's 90nm.
I have a 4600 x2 installed, and a 3200+ in the closet.
The heatsink on the 4600 x2 is a lot bigger unit than the 3200+ came with.
It has the heatpipes and bigger fan, probably the same thing that your Opteron came with.
Well, there you go. I guess the old fan and heatsink will work fine.
I don't know why there is such a difference on mine though, but there is big difference.
I suppose the best thing is try and see how hot it runs, if it doesn't get hot, then you are good!
I think the fan from the 3200 will work just fine, the heatsink that came with my 3500+ was pretty generic and crappy looking, the one that came with my 6000+ was NICE for a stock and had heatpipes and everything...I put the 6000+ sink on the 3500+ and it barely lowered the temps at all(using the same fan both times) I think it will work it just run a little hotter than ideal.