My 9700pro has 2.8 on one side and what looks like 2.3 on the other... I never really checked on that because either way the 2.8 is gonna limit me. To tell write down the code on the memory chip, goto samsung.com, and look it up.
As for the difference, lower rated memory is faster. 2.8ns mem runs at 357MHz, 714 DDR. 2.3ns runs at 434MHz, 869 DDR. To find these numbers for yourself, divide 1000/NS.
--Illah