I've got a 3200+ in my wife's system and have it only at 2.4GHz. 2.5GHz fails Prime even with voltage boost. Not the motherboard or any components because I can hit 300MHz system bus by lowering multiplier. My X2 3800+ is only good to about 2.6GHz with voltage. Luck of the draw. Still, I feel lucky that my CPUs are such great values.
Just remember that overclocking is very YMMV and for everyone able to hit 2.8GHz with their 3000+ Venice, probably five others can't even get 2.5GHz. Just the nature of the beast.