The default clock speed of an XP 1500+ is 1333 MHz not 1150 MHz.
It really sounds like you have the FSB set incorrectly. The multiplier for that chip should be 11.5, and since you're saying that it's showing up as 1150 MHz, you must have the FSB set at 100 MHz instead of 133 MHz. Setting it to 133 MHz will give you 11.5x133 MHz = 1533 MHz, which is the correct clock speed of an XP 1800+. I'm not too familiar with that motherboard. You may have to move a jumper to change the FSB from 100 MHz to 133 MHz or you might just be able to do it through the BIOS.