This is a prebuilt computer; I'm pretty sure its a socket 478/400mhz bus speed/northwood core. The motherboard, i'm not sure, I'm guessing sony built it in house because there's no company name/brand name stamped on the board.
Some online sources say that the processor is upgradable, some say it isn't. I have a good feeling it is upgradable, since it looks like other models in its class used the same motherboard.
As for heat, I'm not too worried, pre-built computers usually have a good airflow/cooling setup -- or at least mine does.