All Athlon64 Single core and Athlon64 x2 Dual core for AM2 Socket fits in.
Socket 939, AM2+, AM3 Cpus are not compatible due to the following reasons:
a) Socket 939, AM2+ and AM3 has different amount of pins. The allocation of pins are different from AM2 CPUs. You can not physically fit them into AM2 motherboard.
b) Dell E521 Motherboard doesn't have enough amount NVRAM to support the latest CPUs.
c) Dell do not provide BIOS update due to the above reason.
d) The memory controller of E521 motherboard is not designed for modern AM2+ and AM3 CPUs.
These are the general straight forward reasons at glance. The other reason includes the BIOS that can not control the Voltage.
So, You CAN NOT install ANY AMD Phenoms, Opterons etc except for AMD Athlon64 Single and all Dual cores. All Windsor family of Athlon Dual core can be installed. Kuma family Dual core is not compatible. Phenom Dual core is not compatible as well.
ALL Athlon and Athlon Dual core AM2 socket including:
AMD Ahtlon X2 6000+ Windsor Edition (125Watt, 3.0 Ghz)
AMD Athlon X2 6400+ Windsor Edition (125Watt, 3.2 Ghz)
AMD Athlon X2 6400+ Black Edition Windsor Edition(125Watt, 3.2Ghz)
There are little notes about installing the following CPUs. These CPUs doesn't work if you do not configure the system correctly as they came out AFTER the last BIOS released from Dell. Please go back to the content for the instructions.
AMD Athlon X2 5000+ Black Edition Brisbane Edition (65Watt, 2.6Ghz)
AMD Athlon X2 6000+ Brisbane Edition (89Watt, 3.1Ghz)
ALL OF THE COMPONENTS IN THIS WEBPAGE IS TESTED WITH AMD 6400+ INSTALLED.