Supporting the Quad would also mean supporting Single-CPU-DualCore G5s which are not liquid cooled and might be a bit less ineffective than the current 2-CPU-SingleCore G5s.
Supporting any given PCIe in a G5 is just a matter of providing the init code (won't be able to show OF or use OSX)
Your X1000 allready has 2cores and might in reallife allready be faster then an X5000 (regardless wether the OS uses 1 or 2 cores)