I have half-dozen Dell CS24-SC.

CPU is currently L5420

I have some X5492 and would like to use them in the CS24-SC. However, it does not appear to be compatible because it will not boot up beyond displaying a black screen and white cursor. POST doesn't start.

Is this CPU incompatible with the CS24-SC-If not, what is the best CPU upgrade it can handle? Or, is this a BIOS issue? 

Any help would be great,