New mboard + cpu dont do much, plz help.

Just bought Athlon850 and a GigaByte GA-7ZM mobo as an upgrade. Problem is, not much happens when i switch it on. I'll try and describe as best as poss.
Power up and cpu fan kicks into life, HD starts to spin and the HD access LED is on. In fact the first few seconds feel and sound 'normal' for a machine kicking into life.
But as i would expect the monitor to start displaying its stuff, no screen appears and no beep(s) from the speaker. Thats as far as it gets, the monitor remains on stanby as it would for no video signal. No more drive access occurs. Nothing, it seems, but the cpu fan spinning.

Some additional info: Athlon approved PSU, G400 32MB vid card, 128MB RAM.

Some things i tried so far is replacing the vid card, but is exactly same prob. I have disconnected any extras (CD drives etc) so only floppy, HD, ram, vid card are hooked up. No joy.
Any ideas on what could be causing it to stop like this? Anything I can try or could have missed?
(Is the mobo just likely to be banjo'd?)

Thanks in advance for any help.