I'm guessing surround if you want optical. That's not loads of money so maybe something along the lines of the logitech sets unless you wanted to ***** a bit more cash either now or in the future and do it properly?? Otherwise 150 could get you a simple amp (unless you already have one nearby) and a reasonable pair of speakers, especially if you can do 2nd hand
Logitech Z5400 (last gen model) I've litterally just got mine last night and they sound amazing, even hooked up to the onboard sound. I am using them via optical, got them for £133 delivered, THX certified, 310watts RMS with a sub that could kill a man. Finding them is bloody difficult these days, pixmania haven't got any more, found these on ebay, brand new.