I don't think there's anything inherently wrong with the suntour stuff that came on the bike. It may be damaged, but should be able to be made to shift very well with a little tuning and straightening. Pictures of the drivetrain would help...but to get you started, the rear derailleur cage (the jockey wheels are in a cage that the chain runs through) should remain parallel to the wheel and cogs as it tracks from large rear cog to small, and visa versa. The cable housing (the tube that the cable passes through) may have congealed grease or rust keeping the cable from running smoothly. A replacement rear cable and housing will probably run you about five bucks or so...