Best graphics card for 1680X1050 resolution?

the title does not say it all, a 4870x2 will outperform a 4850 on 1680 x 1050 res but it may be overkill in terms of return on your dollars. Yet the 4850 may give you solid FPS on high settings in most games with no AA and save you plenty of cash.

Do you want AA turned all the way up?
Will you flip out if some games like Crysis are not maxed out?
Whats your power supply?
Will your CPU bottleneck the card?

^+1 for both...
But in general, the 4870 1GB/ GTX260 core 216 are the best options for that resolution and can easily hold up until newer gen cards are released...

Yeah i think a 260 is really good for that resolution, but as CT615 gave four excellent questions in his post that you should answer for your own benefit.

to make it future proof go for a GTX275 or HD4890 if u can afford or a GTX260 216Core is good enough

I have a 4870 (512mb), and I'm rocking 1680x1050. I was even able to play most games I have maxed out at 1920x1080 before I moved to Japan, and wasn't able to take the big screen. So, the 4870 would be nice. The GTX 260 would be around the same, and anything over that may give you a little more life, but I'm not sure about how much more.

Like how much more life did a 6800 Ultra have over the the regualr 6800GT?

get the 4870 (or 4890) 1GB, it'll rock the 1680X1050 res, and even rock 1920x1080 if you plan to go higher in the future...

As you probably noticed, I'm new to most of this stuff. In response to the questions,

Do you want AA turned all the way up? Assuming that this makes it look better, yes.
Will you flip out if some games like Crysis are not maxed out? I wouldn't fip out, but I'd like to be able to come as close to max quality as possible on most games.
Whats your power supply? 750 watts.
Will your CPU bottleneck the card? Not sure exactly what you're asking.

@ Alsalceion: The 'bottleneck' question should read: What is your CPU?
The graphics card does a fair bit of the work to produce an image but it needs to be fed with data, from the CPU. So a fast card will be held back by a slow CPU because it will always be waiting for data while a fast CPU will be held back by a slow card because it will always be waiting for the card to catch up. In either case the result will be a system that is slower than it might otherwise be.

As a personal preference, I aim to have a system like the first one, where the GPU is a little faster than the CPU.

Tell us your CPU model!!
-Since you are turning AA all the way up and you want crysis maxed out, you are going to be looking at a 350-500 dollar graphics setup   Especially crysis at 1680x1050 with 16xQ AA, you are going to need something really powerful, unless you have the most powerful core 2 dou, quad, or a an i7 then your processor will be bottlenecking you anyway.

This computer isn't a reality yet, so I'm not yet sure whether the cpu will be a

Intel® Core™ 2 Duo E8400 3.00GHz 6MB Cache 1333MHz FSB

or a

Intel® Core™ 2 Quad Q9550 2.83GHz 12MB Cache 1333MHz FSB.

You've been really helpful so far.

a GTX 260 or 4870 will be a good match for your monitor and CPU.

Most current games work best on a fast dual core, but more are using the extra cores of a quad so for a new build I'd get the slightly slower quad.
As an upgrade to an older system the GTX260 or HD4870 is fine, but less good for a new build.
I'm not suggesting you sell your body parts or cut down elsewhere, but look towards the GTX275 or HD4890 if you can get either within budget.
Like the quad, the faster cards are geared towards future demands rather than current needs.
If you can hold off, the new DX11 stuff should be hitting release around October, perhaps you might want to wait until then and re-appraise the situation when some proper facts, prices and benchmark numbers are out.