thats not athlon XP's in general. as praetor said in o/c 101
"Naturally having a good CPU is important: some CPUs are more friendly to OCing than others. Generally, one, two or three models back from the latest and greatest is an ideal OCing processor"
the athlon xp 1800+ is the same as its counterparts, simply underclocked.
therefore, it was extremely easy for you to reach the higher clocks.
for example, i was able to get my X2 3800+ to 4600+ speeds without an overvolt. thats because all athlon X2's are designed to run at 2.4ghz, and then they downclock some of them and sell them for cheaper prices...
again "Naturally having a good CPU is important: some CPUs are more friendly to OCing than others. Generally, one, two or three models back from the latest and greatest is an ideal OCing processor"
my X2 was the last in line, the slowest X2.. and i got it to be faster than the fastest X2 on the market

of course, its no FX processor still.. lol