yea you could. the older steppings don't overclock as well but I would guess 1.8 ghz wouldn't be to difficult but you may want to increase vcore a little if it gets unstable. I would recommend not going past 1.8v unless you have top of line cooling. The more you raise vcore the more heat cpu will produce.

They had a guy that posted some benchmarks of a xp 1800+ on another site (don't remember stepping) overclocked past 2.7 ghz. I would say thats definitely worth upgrading