Going from a 2.8 to a 3.0 will be a rather subtle difference. But generally, you should get quicker acceleration. Your cruising engine speed will go up proprotional to the increase in final drive ratio. Your speedometer will also be off a bit. The NPD catalog used to have speed calculators to help pick the correct speedo gear. Give it a look.
Theoretically, you should see better acceleration, but in reality, you won't see much difference at all; 3.00's are still a very shallow gear. Slightly higher rpms at cruising; probably nothing too noticeable. You should find it rather difficult to break traction with 3.00's and a posi if you are running a small engine.
You won't notice much of a difference in accelaration, 2:80 is going to give better fuel economy, slightly.In my opionion i would go to a 3:50 posi rear, that will certainly make youre pony take of faster/forums/images/icons/smile.gif