my home athlon xp, the mobo only support 333MHz ram. i fix it with 1G DDR400 (OC the cpu and bus speed some more) and it run for a year plus already, but still alive and kicking. 

for DDR2, i never try b4. but can see alot of user suffering some issue when they plug in higher speed RAM. but was solved when the timing was adjusted perfectly. maybe their pc will blow after sometime, shuld pm them after a year or 2.