First of all, no one can really tell what communism would be like in practice, because there has never been a single country that really tried it. All systems that have been called "communism" were in fact socialist, if anything.
The situation with capitalism is similar to a certain degree. As far as I know, there has never been a truly capitalist system, strictly obeying to the concepts of capitalism.
Take america for example. Many people would say that America is truly capitalistic, while in practice, it's far from being so. For example, they'll stick to capitalistic "free market" ideals only where it suits them. When it doesn't, they'll be using protectionism (subsidies, import duties etc. etc.) instead - and people will consider it a good thing, nobody would cry "Hey, that's against the concept of capitalism!"
Especially these days, with emerging nations like China, India etc., I believe the fastest way to ruin the USA (or any other wealthy western country) would be to actually enforce true capitalism.