You might want to check the thermal paste.. has it enough> don't over use it tho..
I'm sure that processor is not worth upgrading and I don't think you can go further much..
You can upgrade your ram to 4gb ddr 2
The graphics card is a pretty good one also.. I mean overall the system is actually pretty good.. I don't know what you want to do..
You'd need a new mobo.. cpu and memory for a total upgrade but I don't know what you want to do.. it's a good system already
but you didn't point out how much ram..