When you do upgrade, there is only going to be a limited amount of room between the tire and wheelwell.

The first number is width - the 2nd the height of the tire expressed as a percentage of the width. Generally, if you increase the diameter of the rim, you are going to have to decrease the height or it will either not fit without rubbing or throw off your speedometer.

Do a little browsing. I went here:
http://www.tirerack.com

And it gave me 205/60/R15 for my 95 Contour SE. Hit the links for 16/17/18 and it appears to be suggesting that I could fit the following:

205/55-16
205/50-17
215/40-18

Just thoughts.

Mike