Depends on your budget. Any of the Big Four's 600 multis would represent a big step up in power. The Triumph 675 may be the most capable bike in that class; since it's not a race bike the extra 675cc isn't really cheating. I'm really partial to the new 1050 Tiger, one of the most appealing sport touring bikes around.
If nimbleness and handling mean more to you than sheer power, the Suzuki DRZ400SM is a quick, nimble motard. The KTM Duke singles are more of the same, witht top-quality components, light weight and very quick handling.