I upgraded my 667 RAM to 1066 (running at 800) EPP, and I see no appreciable difference. The only gains to be made are in tighter latency timings. Some members of the forum, who installed the same RAM as myself, saw their CAS timing go down to 4 (from 5). They DO see a noticeable difference. Unfortunately, it seems to be a craps-shoot as far as what your mobo will support as far as timings. Some people have gotten the tighter 4 timing, while the rest of us have remained at 5 and seen no improvement boost. Since Dell has chosen to keep latency timings locked in the BIOS, there is no way to manually change this setting.