Rendering using either software or Mental ray generally taxes the processor more then anything, so unless your scene is using a lot of RAM or your going to be rendering with hardware then upping the RAM and/or changing the graphics card wont really do much. And if your using 32 bit OS then im pretty sure that a programme can only access 1.5 gig of RAM so upping it wont do that much.

Ive got that processor in my desktop PC and ive overclocked it to 3.2 gig with watercooling, and its running nice and stable, I can go higher (ive had it up to 3.8 gig but it became very unstable) so that might be a way forward.

Im not too sure but if you check the specs of the board it might be able to use a quad core, there pretty cheap now.