What is the fastest single card?
In terms of raw mining performance, probably the 7990 (though it's actually a dual-GPU card). I think the 290x is the next one down, but it's also pretty expensive.
The 280x is the best value for money card for mining at the moment from what I've heard. It's up in the ~700KH/s range while being significantly cheaper than the 290x.
This ^
280x will give probably the best hash for the money on purchase (unless you can find a good deal on 7990's). But bear in mind power usage. 2x 280x will consume more power than a single 7990. Depends on how much somebody wants to outlay in one go, as a 7990 can run double the cost of a 280x.
The 7990 when running at 1300 KH + get hot. Very hot. No way on earth they could be inside a case mining at 1300+ without water cooling. Which of course is extra expense. I have two, which Ive had to undervolt slightly, sat in open air, with external large fan blowing ambient air of about 12 C (53 fahrenheit) at them, and they still sit at 75 C. in a case without water cooling and they wil reach 100 in minutes. So unless you plan on having them in open air (and they are very loud), then 7990 may not be for you and 2 x 280x would be better.