If memory serves correctly - and a quick google search didn't give me an answer - the Doom 3 engine (aka idTech4) was not multicore CPU capable. That was first added to the idTech4 engine for Quake 4, a year or two after D3. Assuming reboot gets multicore ability for PC, that enhancement alone should make D3 a whole lot smoother - maybe take some load off the GPU here and there.

I played the hell out of Doom 3. Very dark game, both appearance and story. Not sure what to expect with the reboot but I will definitely dig in.