As noted, why not just tap the frequency straight off the motherboard? It's right there. But whatever works for you.
(And also, just have to ask... the specs for the propeller say "Clock Modes: (a) External crystal 4 -8 MHz (16 x PLL) ( B) Internal oscillator ~12 MHz or ~20 kHz © Direct drive". I assume you're going to be dividing down that 31.3344 Mhz input frequency if you're planning on using the PLL?)
(And also, just have to ask... the specs for the propeller say "Clock Modes: (a) External crystal 4 -8 MHz (16 x PLL) ( B) Internal oscillator ~12 MHz or ~20 kHz © Direct drive". I assume you're going to be dividing down that 31.3344 Mhz input frequency if you're planning on using the PLL?)