A soundbar, by its very definition, can't deliver true 5.1 - 5.1 consists of 6 speakers, two fronts, two rears, one centre and a subwoofer. Some attempt to create a false surround sound by having lots of speakers built into the soundbar, some of which are designed to bounce sound off the walls so it appears to come from behind you. In my experience, these aren't anywhere near as good as a proper surround system.
Any speaker package you buy will require amplification. If you can't have another box on your rack, your best bet is to try and find an all-in-one box where the amplification is housed in the subwoofer. Unfortunately I can't think of any off the top of my head.
Also, if you're connecting up with an optical connection, there's no need to worry about the HD audio formats as an optical cable doesn't have the bandwidth to carry the HD audio signals - the best you will be able to get is standard Dolby Digital / DTS.