How much are you willing to spend? What you are saying makes plenty of sense. I've seen Logitech and Creative speakers as well as plenty of other brands that support what you want. You could really just buy any cheapo 5.1 system from WalMart for example and it will likely have stereo input. Although it won't run in true 5.1 unless you use optical out from the 360.

http://www.newegg.com/Product/Produc...82E16836121006

Something like that will work. It comes with an adapter that will take the L/R (red/white) jacks and make it into one 3.5mm jack for the speakers. My friend has this kind of setup for his 360 and it sounded good.