M2N-SLI Deluxe won't start

I'm having problems getting my PC working again. I set up the computer last fall from different parts and the computer worked fine. Some time ago I was normally using the computer (only Firefox and iTunes running) when suddenly the mouse stopped moving on the screen. Seemed like the computer did take commands normally since I could shut down iTunes using the keyboard but the screen was stuck. I shut down the computer and tried to restart it but after this the computer seemed to start normally, only the display says "No signal" and shuts down. It looked like the graphics card had died and since it still had warranty I was able to change it to a new one. But the problem is the same with the new graphics card. I connected my display to a laptop and it works fine. I'm starting to think the problem is with my motherboard. When I start the computer the motherboard's led lights up, and all the fans (even the one in the graphics card and processor) start running. Everything looks fine when I look inside the computer but still nothing comes to the display. The motherboard won't make any beeps when all parts connected. I took the graphics card out and still no beep. I took both of the memory sticks out and still no beeps. Does this mean the motherboards broken? It still has some warranty left but I hesitate to send it back to the seller since they charge pretty much from sending working components back for warranty.
My setup:
MOB Asus M2N-SLI Deluxe
CPU AMD ATHLON 64 3500+
RAM Buffalo 1G DDR2 PC2-4200 x 2
GPU CLUB 3D GF 7300GT PCI-E 256M
PSU Nexus PSU real silent 350W

I'd be very grateful if anyone can tell for sure the MB's broken or can give advice to fixing the problem!