Depending on the size of the room for the HTPC, you may want to consider just a decent set of PC speakers. For movie viewing in my old 12'x15' room, my set of Logitech X-530s 5.1 surround was more than enough. For something larger, though, you'd probably want to go with a full 5.1 receiver Home Theater in a Box (HTIB) set up. Unfortunately, those found for around $150 generally are not worth the cost.
I know it's outside you're desired budget, but if you've got a larger area to cover, then the least I'd go with would be something like this
Onkyo HT-S3500 5.1 system.
-Wolf sends