A Sky+ box uses a twin LNB on the dish with two cables to the box.
It might be best to use a Sky or private aerial fitter but it is possible to fit a new LNB yourself and they are not expensive from Maplin Electronics or Ebay. Then the new cable can usually be run through the same route as the old one.
You won't need a new card but you will have to ask Sky to upgrade you to a Sky+ service - the good news is that there's no extra cost to your subscription.
Hope this helps.