I want to order an Inspiron 530 with the Core 2 Duo 6550 (1333 FSB). I would like to spend a little extra on upgrading the RAM but I am unsure which option to go with. Is 2GB @ 800MHz better than 3GB @ 667MHz.
Depends on whether you use more than 2G or not. If you aren't using more than 2G, adding more memory will not help you and the extra speed will help slightly. Personally, I would choose 3G. I've measured the difference in performance between 667 and 533 at about 3% (one core loaded) to 6% (two cores loaded). But that was on an Intel E6600 system so your mileage may vary.