I own a T6100.Its superb for 5.1 movies,games and music etc.The bass is slightly over powering but u can adjust the bass level in subwoofer to comfortable levels.Mid and high frequency sounds are crystal clear.There should not be any vibrating parts in your room because that will create noise due to heavy bass.
Its highly recommended to buy a dedicated sound card if you are in onboard audio.A 24bit basic one would do the purpose.Because when I used it in onboard audio I felt it unattractive.I bought a creative sound blaster 5.1 vx then I could see the difference.
If you have sound card go for T6200 @5k
If you dont, then go for T6100+souncard @4.5k or T6200+Soundcard @6k
This is my 2nd Creative speaker system.The old one (inspire 4400) still works like a charm even after 5 years.I dont know about Z506 but I have seen other altec lansing and logitec speakers.From my experience its creative all the way.Even my mp3 player and headphone are creative - Creative Muvo T200 and EP 630