Any 1080p TV should also be able to display 720p, so if you have a 1080p set, and a 720p game, it shouldn't be a problem to have the game show up in 720p.
As for just getting a 720p set, that's up to you. As time goes by, more and more games are likely to be made to support 1080p. Only you can really decide if you're willing to forgo all of those future games, or at least the ability to play them at full resolution. I should think a 1080p set today is going to be cheaper than a 720p set today, and then a 1080p set 2-3 years down the road, but it's entirely up to you. Do whichever makes the most sense to you.
i have a question, i just bought a ps3, i went a good tv for it, now i know some games are in 108p resolution but most are in 720p..now if i buy a tv with 1080p will the 720p still come out in 720p or be downgraded to480p?...also, if i dont care about the few 1080p games should i just buy a good 720p plasma tv?..theyre like half the price!