yes you can upgrade cpu to i5 or i7 Intel 3rd gen cpu such as i5-3570 or i7-3770.  a -k processor such as i5-3570k or i7-3770k would work too (I had tried a 3570k before) although Dell bios would not support cpu overclocking.  both i5/i7 have 4 cores which are better than the 2 cores in i3-3240.  In addition a ssd would speed up OS boot.
I have a Inspiron 660 and have upgraded the RAM to 16GB to enable faster speeds, is it viable to upgrade the processor too (  Intel(R) Core(TM) i3-3240 CPU @ 3.40GHz 3.40 GHz ). Any  help appreciated.
Inspiron 660 upgrade