Here's what happened in the last few days:
- got this AsRock 775i65G v 2.0 used for a bargain and to my surprise it came with a Core2Duo E6550 still in the socket. Nice bonus, I suppose.
- I have a Core2Quad Q6600 and a Pentium D 945 as well.
- I began tinkering with it, swapping processors and video cards (a GeForce FX5600 and a GeForce 6600GT AGP)
- Had several boot issues so I began thinking it could be a poor PSU fault (see this thread)
- it turned out it could be a RAM issue and indeed, when I swapped a Crucial CT12864Z40B.S16TY 1Gb DDR400 module with an Adata AD1400001GMU 1Gb DDR400 one, all went smoothly.
- upgraded BIOS to the latest 3.30 version, got Windows 98SE installed, even played some games and it seemed to work perfectly.
- as I wanted to have 2Gb of RAM for a WinXP dual boot, I got a Kingston KVR400X64C3AK2/2G kit of 2x1Gb DDR400 modules.
- installed one of those and... the system never booted again. Blank screen, no beep, no nothing. Only fans spinning and the optical drive powering on.

I've been trying everything to get it to work again: unplugged everything but a PS/2 keyboard and the monitor; tried both video cards, via DVI and VGA; removed the GPU entirely and tried onboard graphics; tried all three processors and all four RAM sticks in different combinations; removed the mainboard from the case with nothing but the 20 pin, the 4 pin and the speaker plugged in; cleared CMOS, removed battery and power dozens of times.
Nothing.
The only reaction I can get is three long beeps when I power the board on without RAM. That's it.
Never seen a video signal again. Never heard any kind of beep again (other than those three when no memory is installed).

The last thing I could try would be another PSU but I'd have to buy a new one as I need my main PC for work. I'm afraid it could be another failure anyways, as the system was indeed working perfectly a couple of days ago.

Do you think this mainboard is dead?
I really don't know what else I could try.