Heya,
1440x900 to 1680x1050 is not a huge jump. You have to consider how you `see' things. You see more horizontally than vertically. And the pixel difference between those resolutions is not that big vertically at all, 150 pixels. But horizontally you gain a bit more (240). That's again, not a big gain. You will notice most modern cards (like the 4850) perform perfectly fine in games, new games, at 1680x1050. You won't see a huge performance gain going down to 1440x900. But if you were to see the two side by side, you probably would only be able to pick out the one at higher resolution just because things were a little tighter on the screen and slightely less jagged pixelated straight lines here and there. Ultimately though, they're not that different. Jump up to 1920x1080 and again, you only jumped up by 180 pixels vertically. But horizontally you gain nearly 400 pixels. 400 pixels is a lot in terms of what you get to see. Look at 800x600 compared to 1280x720 for example. Huge resolution difference when playing a game, and that's another difference of about 400 pixels. Going from 1440 to 1920 is a big difference in how the game looks, and how much real estate you have to see.
Higher resolution also results in less pixels being seen on every little detail. At low resolution, every straight line will be jagged. At higher resolution, you will not see that as much. And if you add AA to the visual effect, you get no jagged lines, which is beautiful (more picturesque and less like a computer generated pixeled 3d image).
However, I will say, I think 1440x900 is a good gaming resolution for someone looking for a budget system that can play all games at that resolution and do so really well. It's a good gaming res in that sense. Generally you can even turn on AA on this resolution, it will look great, and still perform very well.
Very best, 🙂