Definitely start by flushing your cooling system and replacing with the proper mixture. Be sure to open the little burp screw when you do it (near the coolant inlet/outlet fitting...I don't know whether coolant is going in or coming out...I'm dumb)
Anyway, you should also check your fan (I assume it's working...but you can check anyway). And as Steve said, check the water pump and the thermostat. You can check thermostat operation by placing your hand on the inlet/outlet tube and waiting to feel the rush of warm water surge through when it opens up. Look at the temp gauge when this happens. Is is somewhere near the middle? If it's getting into the hot side, and you haven't felt that water move yet, your thermostat isn't opening up. Get a new one. Cheap easy fix. Oh, and you can test it really easy when you flush and fill the system. Take the upper hose off. Take the thermostat housing off. Take the thermostat out and place it in a pan of water. Heat up the water on the stove and monitor the temp with a thermometer. It should open at about 170-180 degrees. If it doesn't, replace it. Or you could just replace it, cuz it's cheap, and skip the test. <img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/smile.gif" border="0" valign="absmiddle" alt='

'>
Eric