Unless you get a really cheap deal or you plan on using the video card for things other than mining (games), it's probably not the best idea to be buying hardware right now. If you've got money to burn and you want to speculate, go for it. Even if the current difficulty stayed the same (and it will likely keep going up), it's going to take quite a while to pay off *any* card at the current conversion rate.
imagine my desktop died and i need a new one anyway. imagine i pay 732 and now i have a working desktop plus 632Mh/s. looks like it will take 16 months to break even if rates stay the same. really it was 311 extra to get a pair of 5850s instead of a crap card, so 7 months.
then again imagine i'm in it for the sake of contributing, and would keep mining even if i took a 50% loss on the power bill. yeah it may not be profitable but my point is i fall into a category somewhere between "really cheap deal" and "plan on using".
speculation is fun too. but i'm not selling the farm.