Cowboy Bebop is an excellent anime series. The movie was good too.
Obviously the best soundtrack for a Cowboy Bebop type campaign would be the many Cowboy Bebop CDs. The show had an eclectic mix of everything from bebop to jazz to world music. The show's opening theme, Tank, makes for great chase music.
Bebop does fit the mold of the western quite well, and bounty hunters are referred to as cowboys frequently during the series.
WEG D6? Nah. I think D20 would be the best fit, if for no other reason than feats. Feats allow the kind of customizing that brings characters to life. The same could possibly be said of the GURPS system, but you'd have to buy a slide rule and a graphing calculator...
Crest of the Stars completely and utterly sucked, in my opinion. :\
I plan on buying D20 Future, but that will probably be my last large rulebook for a while, seeing as how my group has dissolved due to people moving away, the younger ones going to college, etc.
Moridin: Color me intrigued.