dadstank, I'd vote for green chromis. they're relatively cheap, nice color, and very active. Once they're in your tank, they'll live forever, but for some reason (probably collection methods, unfortunately) I always lose about 50% of the ones I buy within the first week. Happened twice from two different LFS. Fortunately they're only $4, but still sucks.
Regarding the tang, I have a yellow tang in my 75, and I've had it for about 4 years IIRC... it's been doing great. However, I do feel that it'd be happier in a 6' tank, very active and fast swimmer. This isn't Tang-to-Tang, but I tried to put in a Heniochus and the yellow tang wasn't havin it... chased the thing non-stop so I had to return it. Leaves the Coral Beauty, ocellaris clowns, and chromis alone. I dunno!