What features should I look for in a surround sound system?

I need to upgrade from an old pro-logic setup that I have owned for 15 years. I don't think I need a 7.1 system, but a 5.1 system would at least get me into this century. Are these all-in-one systems any good or should I buy a new receiver? I have a good set of main speakers, but I do need to get new surrounds since my old ones were in wall units that stayed when I moved. Can I use the main speakers that I already own with an all-in-one system and just use the surrounds that come with the system ? A full range speaker would have to be better than one of those tiny speakers, right? What other features are a must have and what can I do without? I plan to hook this up to a new 1080p TV. The TV has HMDI and component video inputs.