G.SKILL PC2 4200 and P5B Incompatibility?

Yesterday I built eight systems for our church. All of the systems
had either ASUS P5B (bios rev. 1102) or P5B-E (bios rev. 1002)
motherboards and EVGA 7000 series NVIDIA graphics cards. Two of the
systems had G.SKILL 2 GB (2 x 1GB) DDR2 800 MHz (PC2 6400) memory
[Model Name: F2-6400CL5D-2GBNQ]. The remaining systems had G.SKILL 1
GB (2 x 512 MB) DDR2 533 MHz (PC2 4200) memory [Model Name:
F2-4200PHU2-1GBLA]. The systems with the 533 MHz would not display
video on boot-up. These PCs would not even beep during power-up which
leads me to believe that the motherboards were not even conducting
POST. I tried several of the 533 MHz modules with the same results.
I even tried explicitly setting the memory timing in the bios settings
to 533 MHz and 2 V instead of the default "auto" setting. All of the
machine would boot normally, however, if I used the 800 MHz memory.
This leads me to believe that the G.SKILL 533 PC2 4200 memory is
incompatible with ASUS P5B and P5B-E motherboards. Is this correct?