I have just installed some new kit: ASUS Crosshair mobo, Athlon 6400, and G.Skill PC2-6400 2x2GB (F2-6400CL4D-4GBPK).
PC will not POST with this RAM installed. I tried all slots, with just the minimum setup (CPU, GPU, 1 stick RAM).
The debug code on the Crosshair says "DRAM DET", lights and fans come on but that's it. Also, with 0 or 1 stick installed it just keeps beeping, with 2 it doesn't.
I tried some other RAM from from a friend's Dell and it booted fine. I also tried the G.Skill in his Dell and it didn't work, although this could be because it is also incompatible with the Dell.
The eBay seller I bought it from assures me it was tested and working, and he has decent feedback (107, 100%).
Has anyone got any ideas of what else I could try? Any Crosshair owners also had problems/success with this RAM? Any advice welcome.
Cheers

(GPU: XFX 8800GTS 320. PSU: Thermaltake 580w. Cooler: Scythe Infinity.)

Dual channel memory architecture
4 x DIMM, max. 8GB, DDR2-800/667/533, ECC and non-ECC, un-buffered memory

Specifications


Speed 800MHz DDR2 (PC2-6400)
CAS Latency CL 4-4-3-5
Test Voltage 2.0~2.1 Volts
PCB 6 Layers PCB
Registered/Unbuffered Unbuffered
Error Checking Non-ECC
Type 240-pin DIMM
Warranty Lifetime


I would first off boot into the BIOS Setup with the working RAM and check the voltage. Adjust to 2.0-2.1 if not already set there. It might be down at 1.8.

I see no conflict in the specs.