Black screen after hardware change - what's happening?

Hello everyone.

As I have recently encountered a computer problem I seem not to be able to solve on my own I thought I'd post here first to see if anyone has useful ideas / comments.

I recently wanted to set up a system using two "old" parts from a former computer. Those two parts are a Corsair VX550W Power-Supply and a Radeon4850 GFX-Card.

I bought some new hardware and intended to go with the following system specs:

AMD Phenom X2 550 BE @ 3.1 GHZ

4GB KIT (2x2) Kingston DDR3 CL9 RAM(KVR1333D3N9K2/4G)

ASUS M4A77TD PRO Motherboard

WD 3,5" 7200rpm S-ATA 1 TB

my old Corsair VX550W PSU

my old Radeon HD4850 GFX Card

& some not-worth-mentioning components such as a DVD-recorder.

I then put everything together & connected everything.

Please note that I have been working with hardware for quite some time so if you're interested in contributing to the solution of this problem, for instance asking me whether I correctly connected this and that isn't going to help and the answer is always going to be "yes". I even double checked everything.

My screen remained black without any beep-code (don´t know what this is called in English) pointing to any kind of hardware issues. It is not a cable-issue either, screen does not point to it. It's connected via DVI and the GFX card worked fine with my (very) old MB MSI K9VGM-V and my Athlon 64 X2.

Observations:

- When removing all RAM, the system returns the corresponding beep-code but the screen remains black.

- This MB has 4 DIMM-Sockets (A1,B1,A2,B2). Moving RAM around does not help. Tried all possible combinations.

- Tried different DDR3 RAM modules. Didn't help.

- Performed CMOS reset, didn't help.

- Tried moving GFXcard to a different PCI-E slot, did not help.

The most interesting observation I have made by far is this though:

The Radeon HD4850 (and all other New-Generation ATI cards) have diagnostic LEDs. The 4850 has two, D1601 and D1602. When I put one or two DDR3 modules in the DIMM-Slots A1&B1 (first pair of DIMM-Sockets, orange color) the two diagnostic LEDs keep blinking and the GPU-fan sounds as if it would have difficulties receiving the right amount of current/power (slow-fast-slow-fast-slow-fast..). One of the LEDs serves to indicate overheating (which is total bullshit, its not overheating) and the other LED serves to indicate a problem with the 12V rail. Now I thought ok, thats strange. I tried putting one or two DDR3 modules in the DIMM-Sockets A2&B2 (second pair of DIMM-sockets, black color). If I boot my computer with this arrangement, the result is quite different: The diagnostic LEDs on the GFX card don't flash and the GPU-fan is working silently and steadily. Screen still black though ;)

However out of my personal experience I'd say that the PSU can be eliminated as a potential suspect cause even with the new components 550W should be by far enough.

Anyone had similiar problems? Perhaps something board-specific I am not thinking of?

Regards

Joel