The manufacturing process of this processor is 90nm. It dissipates a lot of heat which is quite similar to Pentium 4. So you need to get the Heat sink that is used in Dell Optiplex Pentium 4 based desktops. If you use the current heatsink, you will burn this CPU(maybe not now but later on even if you have a good thermal compund). Here is the link address for the Optiplex heatsink I used. Notice the heatsink is different from what you have there at the moment (assuming you didn't upgrade your E521 before)
When you have bought this product, you have to unscrew and remove the shroud since the shroud itself won't fit(it is for C521 so that means you can install AMD 6400 into C521 if you have this model). But first, you must remove the shroud of your current heatsink to replace with the new one.
To me, it was really hard to unscrew so i went to the computer shop and asked the guy to unscrew them for me. You will have to push a bit but the heatsink definitely fits to the current shroud of E521(I asked him to do this as well :-) ) This particular heatsink will fit in Dimension 9200, E520, E521, and C521 safely.
Finally, Buy Noctua 92mm fan and attach it at the back of the shroud. You don't need to use any device. You can insert it and the shroud itself will hold the fan (or if not, you can still use the tape to secure it). This greatly reduce both CPU and GPU temperatures.
The result (with the noctua fan):
Idle Temp 26C~34C
Load Temp 48C~55C
Original heatsink from Dell
Cool down the system just about right
Expensive to get if you live outside America. (shipping cost I mean)