I don't think you guys are realizing how limited Proportional Representation would be in the U.S. political system, without changes to the constitution.
President - obviously couldn't be elected via proportional representation. The best you could hope for was Instant Runoff Voting. In theory, this should allow more options, but it would probably take a few decades for someone besides a Democrat or a Republican to win.
Senate - I believe there is some clause within the constitution which effectively prohibits senators from being elected via proportional representation. It doesn't matter practically though, because with staggered six-year terms, you run into the same problem as the president - you can't have PR with only one open position being voted on.
House - The house could be elected by PR without an amendment, as nothing but statute passed in the 50s bars it. But there are a few hoops to jump through. First, even if the law was never passed, or was repealed, it would be up to each state individually to abolish congressional districts and have representatives elected at large. Even if all states did so, it wouldn't matter in most places due to the federal structure of the U.S. States with only one rep (AK, DE, MT, ND, SD, VT & WY), would behave just the same as now (indeed, it would be better in those cases to have IRV). In states with two reps (HI, ID, ME, NH, and RI) the end result would almost always be one Republican and one Democrat. Initially, I think only really big states, like CA, NY, and TX would really be guaranteed to send some third-party reps to congress.
Essentially, the Republicans and Democrats would remain the largest parties, due to their continued dominance of the presidency and the Senate. minor parties would be involved in the House, but we'd be lucky to see them clear 50 seats. Actually, the effective system would be sort of similar to Canada or Britain, a "two-and-some-change" party system.
Actually, there is one way things could get much more interesting. Given PR would have to be done by state, it would make sense for both major and third parties to operate similar to the Canadian provincial parties, which only have tenuous connections with one another. What I mean is, since the Republicans for example will be running a party list in both New York and Texas, for example, it makes sense to have different platforms in both states, in order to cater to local voters as closely as possible. Over a few decades, this could drift so that the New York and Texas Congressional Republican parties are very different ideologically, with a governing majority eventually formed including one but not the other. Interestingly, this would probably take U.S. politics, at least in the House, back closer to what the founders wanted, with people more identifying with their state than a national political movement.