if you pitch yeast on the higher end of the fermentation temp for ale yeast, it will lend more fruity, estery flavors, as opposed to the lower temps. maybe even a stronger alcohol after-taste. fermenting on the lower end will provide a little cleaner, better flavor, and probably help the yeast have a better, active ferment (which is what we all want). depending on the brand of yeast you use (liquide White Labs or Wyeast), they will have a temp range on the vial or pack. or, you can check the web pages of each (www.whitelabs.com, don't know Wyeast's). White Labs has a FAQ's page that you can get some good info off of too.
if your not using liquide yeast, i recommend a good quiality liquide yeast (and a starter if possible, but not required) like White Labs or Wyeast. that is one of the biggest improvements you can make on your homebrew.
hope this helps.