To the best of my knowledge, the ps2 only does virtualzed surround (Dolby Prologic? can't remember) and not true DD or DTS as the xbox does. It's title specific, though; HDTVarcade would be a good place to look for specifics.
As for the cable.... I'm assuming you want the PS2 and Xbox both plugged in at the same time; you'll need an optical switch to select whats heading to your speakers. Most decent/high end receivers will do this, or you can pick up something such as the Pelican System Selector Pro (linky
)which is what I have at home.