You need to contact dell to find out if it is compatible. If not you need to find the model of teh motehrboard that you havem then go to the manufacutrer website and see if the CPU is compatible with that board.
The simple answer is NO.
The maximum CPU that you can get for this PC (Dell Dimension E520)
is an Intel QX6700 Quad-Core 2.66GHz/8M/1066 (SL9UL) LGA775 CPU.
The other CPU’s on the upper level are Intel Q6600 Quad-Core (QMAQ, SL9UM, QXVD, SLACR),
and Intel Q6700 Quad-Core 2.66GHz (SLACQ or QXUZ).
I have a Intel QX6700 EE.
The capital letters in the prentices are the CPU ID numbers,
you can search the web or eBay by this numbers for the price.