I use the old classic theme, I can't stand the new themes, even the bento theme, but I use winamp.
I've found that with the plugin system, and the huge userbase, I can get winamp to play pretty much any music file I could ever want to listen to, and more. Not to mention it takes up way less ram and processor time as other programs as well. On my computer, winamp takes less than 1% processor time more often than not. It fluctuates between 0% and 1% on my computer. WMP takes like, 5%, IIRC, and it eats way too much ram. iTunes, I'm not sure how much processor time it takes, but it takes more ram, and I don't need a program to organize my music. It's called folders, they're built into your operating system.