fishninja, an XP 1600+ runs at 10.5x 133 (266 DDR) FSB-- which works out to 1400 MHz. To get 1670MHz (appx) you need to raise the FSB to 158MHz (316 DDR). That is hard on most systems.
cusimar, I'm surprised you couldn't get your 1800+ higher than that. I have the stock cooler which isn't much more than adequate, and was able to get mine above 1.6Hz without trouble. But as an aside when I say you should be able to get the majority of cores up to 1.6GHz with air cooling I do mean using better than the stock solution.
Yes the XP isn't that O/C friendly (not like the T-bird) but I still think it's a great processor.