When overclocking an Athlon XP, most people can see around ~100-200Mhz overclock with a simple fsb raise(possibly more).
Since the XP is factory locked, and you can't do the pencil trick to open up the multiplyers, it's alot harder to raise the mults by modding it and essentially opening up the bridges for the higher mults. It's not very safe and not guaranteed that you won't fry your chip but it is possible. If you do unlock your 1800+(which is 11.5 multiplyer set at factory), you can probably raise it a couple of notches, but stability is still a problem depending on how far you raise the fsb.
Overclocking depends on the mobo and the type of cooling that you use in alot of cases. For the 1800+, which has been a good overclocking chip, I would expect to get up to 200 or possibly 300Mhz overclock if you use good cooling, but not alot more than that.