Update: Figured I should come back to share my findings. I did the fresh install of Win 8.1 64-bit on that system and it works perfectly, even though not officially supported. I'm actually pretty impressed with the performance, and it seems much zippier than XP on this hardware.
The only issue I've run into was putting the system into sleep mode. It wouldn't wake up from sleep, and I had to cycle the power to get it to boot. After that, the P5Q board settings were all back to default, so I had to reconfigure the BIOS.
Mobo: ASUS P5Q
CPU: Intel Core 2 Duo Q9550.
RAM: 4GB DDR3 1333