Now, one thing I don't know is what RAM speed you have and what the FSB is on the old XP chip, but you might have overhead with your RAM so you could OC at 1:1 without OC'ing your RAM as well.
For example, if your FSB is 133 (533 total bus) and you have PC3200 RAM (200Mhz speed), you have a lot of overhead to OC with.