You can run a q6600 or a q6700 as well as the qx6600 and the qx6700 on an e520 with the latest bios which is 2.4.0. i wouldnt suggest the qx series being that you wouldnt have much use of the features of that processor being the dell locks out overclocking features!!
Dell Dimension E520
Intel Core 2 Quad 2.66GHZ Processor
MSI NVIDIA GTS 450 Single Slot GPU
Corsair GS600 PSU
8GB Crucial Ballistix DDR2-800 (PC-6400)
Windows 7 HOme Premium 64bit