Any new ATI card. My recommendation would be 7950's - they get you the most hash for your money and power consumption so imo they're very worth it.
I'm currently running a few of them and each gets 600KHash minimum on Scrypt coins (litecoin included). There are optimizations if you look around to get 7950's to 650ish KHash with lesser power consumption - imo definitely worth looking at.
uhh, no. As the difficulty increases those cards will not be worth using anymore.
Please check out http://dustcoin.com/mining - if you do some basic calculation, you'll find something like Litecoin mining to be plenty profitable at the moment. Sure, there is always the arguement that once the difficulty increases, it'll no longer be worth it - but do you really think thats the way its going to work? You dont think the market price of Litecoin will adjust to the difficulty of the coin? As scarcity increases, price will also increase. You're allowed to say anything you want and speculate anything you want, but mining simply wont be unprofitable until FPGA or ASIC farms come into play for scrypt, and that wont be for a long while.