I have a 19" WS running at 1440 x 900. The difference between the two cards at newegg is about $100 bucks. I don't mind spending the extra $100 if it will make a difference, but if it won't make much of one I will just get the 320mb version.
If you play at a 1600X1200 resolution or higher, than get the 640 mb version. If you play at lower resolutions, then get the 320 MB version. And since you're running at 1440X900, then the 320 MB one should be fine. You can always use the step up program and get an 8800 GTX in the future anyways...