Both motherboards are fairly basic so i doubt either will have many bios settings that alow for much tweaking for overclocking so it probably would be more confusing than anything else so dont worry about it.
An X3 at 2.8 fails to match a Core2Duo at 3.0 and is around 5-40% slower depending on the game going from memory, so the gap between a x3 at 2.1 and a c2d at 2.6 will be alot more noticeable and should give a noticeable boost in performance for everything you throw at it.