My now 5 year old was previously on Enfamil AR and we did notice that his spitting up decreased, but ...
If you baby has not had an official diagonosis of "reflux" and has not had an upper GI or any other testing -- then you may want to consider a milk allergy. For some reason lately it is so common for pedi's to diagnose "reflux" in babies, true reflux in babies is not as common as we'd like to believe.
More often than not - it's a food allergy. More and more children are showing signs of milk allergy's than ever before, we are learning that babies w/ "colic" are babies w/ milk allergies.
Here's an article from Dr. Greene:
http://www.drgreene.org/body.cfm?id=21&action=detail&...
The date at the bottom of the page is 1997 - so I'd check to see if since that time Prevacid has actually been approved for pediatric/infant use. Our 3rd child was on Zantac for almost a year (before we learned it was a food allergy) and did suffer some side effects from it. Our 4th child was on Prevacid for a very short time, then we did some research and opted not to give it to him (it didn't help anyway).
Our 3rd child was diagnosed w/ reflux, and the Zantac and Enfamil AR helped so much that it masked his allergic reactions and we did not realize until sometime later that he had a milk allergy and his "gut" was suffering (he had allergic colitis).
When our 4th child was born and wouldn't stop spitting up, he had an Upper GI w/ a small bowel follow-through and they found no physical signs for reflux. We took him to the chiro and switched his formula and the relux stopped.
In both cases, it was a milk allergy.
So my suggestion is before you try the AR, try a non-milk based formula like Nutramigen or a soy-based formula like Isomil and see if that improves the reflux.
And of course, you should discuss these options w/ your pedi :)
Good luck to you and your baby!