Get the 22", 1680x1050 - the price/size ratio is about right. You can get it for about $200 - $250.
You'll be amazed, coming from the 17" CRT, and your 8800GTS will most likely be sufficient, too.
And get the monitor first - whatever size you'll decide to get.
It will give you an opportunity to test your 320MB GTS. If the performance is disappointing, it will be much easier to decide to get a new Video Card.
And it will offer you a much better excuse to spend the money on the Graphics Card... LOL.