Your system supports DDR2 Non-ECC memory. Yes, the maximum speed recommended is PC2-5300(667MHz) anything above should cause compatibility issues. Moreover, the maximum memory recommended by Dell is only 4GB using four 1GB modules but crucial states 8GB which is interesting but i'm not sure if it will work. Anyways, if you need a different brand memory then try here - http://www.memorystock.com/memory/DellDimensionE520.html
Good luck with the upgrade. Cheers!!