For a HTPC, 50W should be more than enough, since you're physically very close to the monitor screen, and presumably physically very close to the speakers too. Unless you have an unusually large room, a 50W receiver would do just fine. Good receiver brands include Onkyo, Yamaha, Denon, Harman Kardon, the Pioneer Elite series and maybe NAD. Your choice depends on several factors, such as the feature set you desire(different surround formats), sound quality and budget.