What the sort of consensus here would like to see you get into is a modest to decent receiver & quality speakers that will satisfy you indefinitely. Yes, that goes beyond & involves some more money than a quick recommendation for an HTIB (home theater in a box.)
The reason being those of us with and appreciative of very decent sound systems feel many can come to appreciate it with confidence the quality of sound will enhance the HD experience. The receiver needn't be overly expensive as we try to emphasize that an investment in speakers from makers specializing in them should be around 70% of the budget.
Perceptions of speakers may seem to be that they are expensive. Well sorry, but they can't be both actually quality and cheap. With good choice they will last well for decades. Components change technology rapidly and call to be replaced. Decent speakers don't and basically don't wear out.
With some more specifics on your part, a predictable set of common recommendations will be posted.
I just recently upgrade to an HD plasma tv and am in the market for a new home theater system. Any recommendations. Thanks