Well I understand the following:
When you boot a PC, the motherboard does a "power on self-test".
Every signal during that time is a POST notification.
You say that the beep occurs after this test is already finished?
@USB:
I had various issues with USB peripherals connected on boot.
I am not completely clear on what kind of testing you did;
As I read it you did the following two which did not change the problem "significantly":
- Boot minimal config (no mouse, no additional HD, no disk drive, no floppy, one RAM, no case fans, no peripherals, no networking)
- Boot normal config with no USBs at startup (some USB devices are just problematic; That for me includes my ipod btw)
You could to make it definite that it's not a POST, take out one piece of hardware from the minimal config and check if the bootup process works out like before (try booting no RAM, not video card, no monitor, etc.), thought you should be able to see the ram check if you did disable the BIOS logo.
btw, using 2 brands of RAM together is already risky business in the compatibility department.
Did you try shuffling them around, or is there a recommended setup in the documentation?
However, there are steps I would recommend (do any you didn't already do);
Check all connections especially the power lines of the PSU (unplug, plug in again)
Set your fan speeds to continuous in BIOS (and/or load a fail safe mode)
Check that the BIOS battery is not depleted and/or badly connected
