Ok all, I know that it has certainly be reported times and times, but I did not find any answer.

I have an "early" imac 20" with a 2.0Ghz cpu. The warranty is about to expire, and I would like to change my dual core cpu with a more powerfull dual 2 core. but the problem is that they all confused me with the name and specifications.
I would like to know what is the cpu name who is in my imac (johna ? memrom ? ...) , what is the socket type (478,479?), what is the cpu type (5500 ? 5600 ? ..) and by which cpu can I change without any problem ? I suppose the more I can put will be a 2,3 ghz ??? but it has to be the same socket type and to not heat too much ?
Any help welcome. Thanks !