You can answer that question yourself.
Are there any games that play unsatisfactory on your MX? If yes, a GTS will improve that (depending in CPU speed).
If not, then sit tight till the GF3s are priced to move.
IMO the biggest difference between the MX and GTS(or pro) is not speed, but resolution. If 800x600 is ok for you, stick with your mx. if you want higher, get the gts.