The unit in question started life as a Presario srs5100nx before I did some upgrades to it.
I'm fairly certain is is AM2 socket. The 4800 CPU I spotted online is AM2 socket and is only $50 but it is OEM (so no heat sink) which is why I was wondering if I could get by with the existing heat sink. But I guess I won't be able to get away with cheaping out on this one. Either will have to purchase one of those units (thanks for the link evongugg), or break down and just buy the CPU as retail in order to get the included heat sink. Either way it will end up being about $100 for the set.