not sure about the nuforce as i'm not familiar with them but the creative aurvana's are good cans for the money - they just sound best at lower to mid volume ranges and not loud. some people use them for extremely budget oriented headphone+mic gaming headsets although for more money you can do better. i'd say the 518 is s till better though.
the m50 (i own one so can speak from personal experience) is a warm sounding headphone with a tinge more bass than normal while still having detailed treble. built like tanks. one negative to them is an incredibly small soundstage which makes them not so ideal for gaming although you could certainly use them for that if you dont mind the lack of good positional audio)
the ad500x is within budget...