Sidewinder was originally written when 3.0 D&D had just been released and there was no d20 Modern. It adapted the 3.0 d20 system for an old west setting.
Sidewinder: Recoiled is the revised version which is based off d20 Modern rules. d20 Modern is a much better fit for the old west genre. It is a well done and self-contained game. I highly recommend it.
There is also Link West, which uses the BESM system version of d20.
There is also OGL Wild West by Mongoose, but I haven't read it.
There is also a d20 Deadlands version. I haven't read that either.