i think Sonic means how long will it last as in being able to play the latest games...as in how long will the components he installed be relatively up-to-date.
I believe that system will last a few years (from what i can say) well until DX10 takes off properly, when all new DX10 gfx cards will be more widely available and a tiny bit cheaper
Your idea on the quad core on the other hands is pretty useless, if you're using your system for home/gaming use, there really is no need for a quad core CPU, you would be paying money for something that you won't use to its full potential.