a speed up question
 

80% of what a C program dose is done by the system libs
so how much of a speed up would I get by upgrading from compiled for i486 to compiled for a i686 would I get ?

and would it matter for the kernel if I upgraded before I built a new kernel?

I would think it all depends on your system really. But to tell you the truth, you probably wouldn't see any difference from the naked eye.