I am looking to try to improve my 2005 trek 1000 and I had some questions about compatibility.

• If I change from a three ring to a two would it effect my shifters or are shifters designed for a certain gear number and ratio
  
• Are all 700c wheels compatible. If I get new wheels will my old cassette fit.
  
• When cleaning my rear cassette can I use bike degreaser or will it mess it up (If it does what should I use to clean it)
  
• I don't have alot of money but What would be the component that would make the most difference in performance? Wheels...crank...carbon bars, seatpost, or forks. I want to upgrade but I don't have the money to buy a new bike.