CPU upgrade for Optiplex 780 sff with pentium.

I know there is extensive coverage on cpu upgrades for this pc, but let me explain. Ive searched all over the dell forums and the problem I have is theres no clear answer. Most people on there are trying to upgrade from a core 2 duo. However the 780 sff I have came with a Pentium E5300 dual core. The problem is that I was either looking to upgrade to a core 2 quad or xeon quad core, preferably the xeon as it is half the price for almost equal performance (and just sounds cooler). On dell forums they said that in order to use a xeon you would need to do a bios mod or update, but this was for people with core 2 duo systems. Does my system having a pentium mean that it already has the bios version to use a xeon, or should I just get a core 2 quad? (Also if upgrading to either would I need to replace the 255w dell psu?)Sorry if this was long.

Edit: Thanks for the help NO ONE!