A quality surround processor makes a big difference, though it's quite subjective (obviously if you don't really care about how your AV sounds, you won't notice the subtle differences that a home cinema fanatic will gush over).
I've had a Yamaha DSP-E800 plugged into my hifi/AV system for years now. It does the job, but over the years I've become less and less satisfied with it, partly because my sources and preamps have gotten better and better (which of course makes the Yammy sound worse and worse by comparison).
A while back I spent some time with a Meridian 568.2 and a Sony 9000ES (a high-end Sony surround processor). They both sounded very different from the Yammy and from each other, and both the Sony and Meridian were a lot better than the Yam. I was, however, a bit surprised at just what was improved with the high-end procs. The big, noisy action sequences, which are favourites of the AV crowd for demoing and judging performance, didn't sound too much better on the high-end procs. Instead it was generally the more atmospheric scenes, where subtle ambient sounds were much clearer and more involving, and really added to the experience. Surround effects were also steered more smoothly and seemlessly between the speakers. And finally, and this applied particularly to the Meridian, bass was much, much better - cleaner, smoother, much more natural.
Dunc