First, I have to say that I have trouble imagining what kind of scenario you'd need to this in.
But, if I had to do this:
.Replace(">=", "GTE")
.Replace("<=", "LTE")
.Replace("<<=", "SFL")
.Replace(">>=", "SFR")
.Replace("<", " < ")
.Replace(">", " > ")
.Replace("GTE", " >= ")
.Replace("LTE", " <= ")
.Replace("SFL", " <<= " )
.Replace("SFR", " >>= ");
Perhaps one of our resident RegEx gurus will come along and make it easy :)