PC Won't Boot (No Fans, No Sound)

I purchased a new PSU and paired it with a system I already owned - plugged it all in without a hitch. When I flipped on the power to the PSU, the power-on LED on the mobo turns on, but when I press the power switch to boot, several LEDs flash on the mobo (as well as a brief "FF" code) before turning off. The power LED remains lit on the mobo, but nothing else seems to be getting power. No additional lights turn on, no fans, no sounds.

CPU: Intel Core 2 Quad Core Q6600 (2.4 GHz)
MOBO: EVGA nForce 750i SLI FTW
RAM: 8 GB total (Corsair XMS2 PC6400 800MHz 2GB DDR2)
HDD: Seagate 750 GB, 7200 RPM
PSU: Ultra 550-Watt
GPU: 2x BFG nVidia 9600 GT
Additional Drives: HP 24x CD/DVD Writer

I upgraded my old PC to an AMD-based set (new mobo, RAM, CPU, GPU), so I decided to use the listed mobo, RAM, and CPU to build a new computer for my father. I left the old items in their current case, replacing only the PSU, HDD, and DVD drive (which are all the items listed above). After turning the PSU on, the mobo indicates that the mobo is getting power. But when I go to press the power switch on either the mobo or the case, several LEDs light up on the mobo and a quick "FF" code flashes, but then the LEDs and the code vanish. Despite that, the power LED on the mobo remains turned on. There are no noises and no fans spin when the lights turn on/off.

I've tried resetting the CMOS, cycled out each piece of RAM, reseated the GPUs, BIOS chip, CPU, etc... I've also tried removing the GPUs and the additional drive to see if they were eating up too much power. I've not yet tried to swap out the PSU (going to try that in the morning when I have access to an additional PSU), but I was wondering if anyone might have an idea as to what might be the cause/solution. Thanks for your time!