Wheels will get you more bang for the buck in any upgrade. Though I wouldn't upgrade your wheels until they are dead or if you can sell them to off set the cost of the new hoops. Rims in the 21-23 range for the wide tires your bike takes, laced to 105 or Ultegra hubs, preferably hand built.
I would upgrade the cranks on that bike first as it looks like a boat anchor. My choice would be a White Industries ENO crank in black, though it is pricey. You could probably keep your bottom bracket with that crank.