you dont need to buy a sram front derailleur, shimano front derailleurs will work just fine with sram shifters.
as for the front chainrings i'm going to assume that you just want to get rid of your big ring. you'll just need to buy a bashguard that is big enough to cover your middle ring and has the same bolt pattern as you current chainrings. something like this
http://www.universalcycles.com/shopp...84&category=58