For £150 you'd probably get better performance out of a stereo system, rather than trying to skimp on multi-channel, 'though £150 would probably get you an 'out of the box' solution from a catalogue shop?
I've got a Creative Inspire 4.1 rigged up on my PC. It's a bit old now but you get a great surround sound for games - and I'm guessing it would work for xbox?