I concur with Hillrider, the easiest way to make your bike go faster on the road is to switch to slicks. If you do a bit of both road and trail, you could try a tire like the Continental Double Fighter. When comparing your gear ratio to other bikes, you have to consider the tire size. a 12 - 52 is going to be a higher gear, if your bike is 700c or 27" than on the 26".
52X11 on a For 700 X 23 / 23-622 tire with 170 mm cranks = 124.2 with a 26 inch tire the gear-inch would be 121.7.
Note that switching from a 12 to an 11, will get you another 10 inches!
Changing the front rings gets you less change per tooth then a one tooth difference on the rear. 109.4 vs 111.6 for a 51 vs 52 respectively.
Play with the "gear inch" setting at Sheldon's gear calculator.
http://www.sheldonbrown.com/gears/