My car is acting up???

Lately my 90 lx has been acting up. The motor will get really warm, but the temp. gauge doesn't show it getting real warm. Sometimes smoke will come out of the motor around the headers and I get a real oily smell. I was told that the smell could be because of the valve cover gaskets. Also my exhaust sounds weird. The driver's side tailpipe sounds normal, but the passenger side sounds kinda muffled, like something is stopped up in the pipe. One of the mechanics where I work said that the cats could be clogged up. Would thisbe causing my problems. Also I just changed my spark plugs, and the driver side ones looked normal, but the passenger side one's were really white on top. If anyone has any suggestions please let me know. Thanks.