Yes. You don't fully need DDR2-800MHz ram, but you do need faster ram. (might as well buy DDR2-800, its cheap.) I have the 6600, so I know the specs by heart. 266FSB times a CPU multiplier of 9 equals 2.4GHz CPU frequency. Your ram should be AT LEAST twice your FSB, so you need at least 533MHz ram. (266 x 2) I just realized you said PC4200 and not PC3200, so the ram is (4200 / 8) DDR2-533. You don't need faster ram unless you are overclocking. Hows that for a change in mid stream?