C.,
I have heard good things about Calvary and Wee Folk in Downtown Frederick as well as Good Shepherd on 7th Street.
I have not used any of these personally (I started my girls in Pre-K at Mother Seton in Emmitsburg, which is a hike, but I LOVE it there). I have friends who currently have their kids enrolled (I know at least one at each school) and they all have nothing but good things to say about where their kids are.
One thing to look out for is the MOMS Club Pre-School Fair which usually happens in the Fall. One of the Frederick area MOMS Clubs gathers all the local preschools in one location for one night and you can "shop" the schools in an environment much like a Job Fair.
Good- Luck!
~ B.