Just got through act 1 of nightmare no problem. I am at about 600 DPS. Is that good or bad for my level?
If you want good DPS just keep your bow within 2-3 levels of you. Rares/Legendaries only.
Magic is fine so long as it puts up good numbers; most of the rares at this level you're spending extra $ for junk stats, and the ones that are completely synergized frankly cost more than you want to spend at this level. Once you hit 60 and can focus on long-term optimization then go rare/legendary only with the stats that you want.