Yep, I'm afraid if you want to push those resolutions smoothly, you'll need more than a mid range card if you want your card to last even a month (maybe a slightly exageration), but that is a very high resolution, especially if you would rather run games at full quality for example.
I think a 4870 might just cut it, seeing as you still seem to be on a budget.
i'm guessing 500 odd dollars for a 4870x2 will be slightly too much
So yeah, I suggest a 4870, and then upgrade with another later when you need it.
I personally am not a fan of SLI or Crossfire though. I don't see the point in the end.
EDIT:
I was going to say that getting a monitor that size and opting for a 1080p edition won't be noticeable over say 1680x1050, and in a sense, it won't...really, but since your sitting so close to it, and you may want like the full experience of lets say a Blu-ray film, it may be worth it.
Just remember that if you are going to be running games at a the full resolution, and you want to be pushing the settings to the max, you may have a problem with some games later on.
Luckily though, since the monitor would be rather small, but has a lot of pixels packed into it, the advantage is that you could probably play games without the need of antialising (or not much of) since you won't notice the stepping effect as pronounced as before, like you would on lets say a smaller resolution on the same sized screen where the pixels are bigger.
Antialising tends to eat up even the most powerful of cards.
