The higher the resolution, the better the games look, but the more GPU you need to run higher frame rates. Its a personal choice to balance performance with how much you're willing to pay for it.
If you're trying get some real life out of this upgrade, look at it this way:
CPU - The Q9400 will last for a long time. That's a sound choice.
Ram - If you're trying to get years you of your computer, 4 gigs would be appropriate, a sound choice.
GPU - The 9600GT is based on a chip that was released in 2007 (8600GTS) and upgraded early 2008 to the 9600GT. So you're already 2+ years into the life of a 9600GT. If you can swing a 5750, thats only 2 months old. It will last a lot longer than the 9600gt and let you run your games at 1680x1050. If you can expect 3 years out of a GPU, there's only 1 left on a 9600gt and its really underpowered to run 1680x1050. I just don't see it as a good idea and you'll be wanting a GPU upgrade long before you're expecting.