just for another voice on this...i'm in a similar situation. i bought a new tank before i had decided whether to go marine or do a discus/cichlid setup. Upon reading about marines i then discovered the option of a sump etc, and by then it was too late. There is no way i'm gonna drill it myself or take it to an lfs.
I don't see it as much of a problem though, as i've got a hang-on skimmer and will be using live rock and the tank's built in filter in the hood for filtration. Should be ample.
I know the sump increases livestock capacity but for a 6' tank i'm guessing you won't be too short of that anyway...