Not to disagree with John's excellent advice, but a different opinion:
I don't go for the whole "clean up your AC and your televisions' picture will improve and your audio will sound cleaner" theory, UNLESS you have some particular interference issues.
But, as someone who does NOT live in an area particularly prone to lightning, I have lost equipment to non-direct hits. I have a whole-house lightning arrestor, but have had phone-line surges cause loss of TiVos, credit card swipers, phone systems, etc. (I have an in-house video production company and have many tens of thousands of dollars in equipment to protect).
Anyway, my thought is that for a home theater system, there's a good compromise between a $20 suppressor and a rack-mount $400 unit. This is what I use to not only protect from AC spikes, but telephone and cable as well:
http://www.tripplite.com/products/product.cfm?productID=2812
It runs around $120.
