do not compromise. if you have the cash, buy what you want. that way you won't have the shoulda/coulda/woulda feelings later.

that said, it sounds like your budget is $500-600? if you can afford a little more a Surly Cross Check isn't a bad way to go. if that's too steep, Kona's Asphalt line has a number of flat bar commuters that are in a decent price range. consider a road bike though, they really aren't that uncomfortable. you don't have to have a 5" saddle to bar drop if you go that route you can't go wrong with bianchi. they're a little more expensive but very much worth it.
i'm going to plug Felt since i have one and i love it. they jumped in price from 06 to 07 and i imagine they're going to jump again, but their Z series relaxed geometry bikes are still pretty reasonable.