Unless you were experienced enough to not need to ask here, then swopping the CPU is unlikely to achieve much for you. As VHG says the CPU is determined by the PC's motherboard so at best you can probably find a faster version of the same processor you are already using. Useful for the cutting edge addicts to boast of, not for much else.
Naturally if you were thinking of building anew, from scratch, the type of site you requested would be useful, but not unless.
Normally one would tend to suggest you have added the maximum amount of memory, made sure your hard drive is large enough to have lots of free space and leave iot there. Maybe if you are into gaming you may want to check the graphics you are presently using, and the type of free slots your PC has for a new card; should you even go that route. But to be honest 'looking to upgrade' is quite a vague phrase, may be useful to work out exactly what you hope to achieve.