Have you looked around in the BIOS at RAM settings? Do you see anything that looks strange or that you don't understand?
Have you reset the BIOS to factory defaults by pulling the CMOS battery on this motherboard?
Your mobo manual says you need DDR2 667 or DDR2 800 of 1.8 volts.
That appears to be what you have, but I am wondering if the BIOS shows anything odd or conflicting.
I assume all parts are new or purchased new very recently and that you are not aware of any of them being defective. If that is wrong, let us know.
I also assume you have tested the motherboard/RAM/PSU with the motherboard sitting outside the case. If that is wrong, let us know.
I assume you have a total of 2 sticks of DDR2-800 RAM, but have not done any specific memory tests on these sticks. If that is wrong, let us know.
I assume you are now using a second new motherboard (Gigabyte EP41 UD3L), a second new PSU (OCZ 600) and the same 2 sticks of RAM (Micron SDRAM DDR2-800) that you used on the old motherboard. If that is wrong, let us know.
I assume both RAM sticks fit into the motherboard easily and that all retaining clips are upright and locked. If that is wrong, let us know.
If you get any beep codes at all, advise.
I think that is a potent and powerful video card. Is it supposed to have any direct connection to the PSU?
The flickering LEDs on the RAM isn't a good sign.