Dude, you have to see what socket your motherboard has. The socket is the place where you fit the CPU and, as such, the heatsink will differ according to the kind of socket. For example, an Intel Socket 775 heatsink won't work in a 478 Socket (Well, there are some which support mutiple sockets).
I am not aware of what kind of AMD sockets there are, but it should be pretty straight forward to find out. Look for it in your motherboard's box.
Edit : I never quite understood AMD processors, but by looking yours up (Athlon 64 X2 4200+) on wikipedia, I found out it is either a 939 or a AM2 socket CPU.
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