I tried some corsair pc2700 with my northwood and I wasn't impressed with its overclocking ability. Maybe it was because of the different platform, but the best it would do was 166 with turbo timings. I've had Corsair pc2400 up to 185 cas2 on an Epox 8K7A. I would definitely unlock your chip. I've yet to get any XP faster than 1750, so your cpu may well be holding you back, and not your ram. I'd push your bus and ram as high as possible first.