I like the FMF Power Core
IV Spark Arrestor, I found it added usable power across the band. Some people find it too loud, it depends on your goals. I ride and race desert, the FMF works for me. I'm running with the stock header pipe for now. But I have friends that run White Bro's E-series and love them! Before I'd suggest a pipe to you, I'd need to know what you expect from it. There is no free lunch! There are top end pipes, low end pipes and a little bit every where pipes, where are you looking to gain? Do you need a Spark Arrestor? Would you like to save weight or money?
Whatever you do, jet the bike correctly for where you ride, I'd never touched a carb before last month and people on this list helped me out. Check the forums and you'll find "how-to's" on replacing the Main Jet and Needle Jet. It's not that hard.
Chris in the Mojave