Personally for the price range you are looking at I'd get a Kona. From what I've seen in the lower end bikes you get slightly better parts on Kona's. Plus I like the looks of them!

For $550 - Hahanna

For $650 - Fire Mountain - I like this one, nice crank, RS fork, Alivio rapid fire shifters, plus you can get it in a cool polished aluminum finish.