i have a '65 Melody Maker too, with two pickups. the single coils had a nice enough tone, but were quite weak, and when the bridge pickup broke (the lug for the height adjustment screw) i decided to try Lollar's melody maker p90's, on a pre-wired scratchplate. they sound great :)
doesn't really answer what an SG sounds like with single coils though. i think it would sound great, myself. i'd get a batwing plate version and get a scratchplate made for 3 strat single coils, could be a great guitar