So how cheap are you thinking?
... and you don't want to just hook this up to your hometheater set? 5.1 to 6 channel direct on your receiver?
If this is just for gaming and not music quality etc., then logitech makes nice sets for a lot of grunt bass . If you're looking for something a little more powerful than the systems already mentioned,
5300s will give you a good sound level (~$160).
Do you have an old receiver lying around? A couple sets of bookshelf speakers and
a dayton sub would give you really nice bang for your buck.
EDIT: haven't used PC150s, but my 64ohm HD280s worked well with my old TBSC.