non booting problems

i've just installed a new motherboard (asus a8n) and cpu (opteron 144) in my old rig because the mobo was faulty (ide port didnt work).

now it wont boot up (it doesnt beep or post), everything comes on but the monitor just displays no signal input and then goes off and the HDD led just stays on for a second and then nothing???

the dvd drive opens and closes still, can anyone help since i've no idea what to do?

full specs list?

is it an Asus A8N or an abit AN8?

Asus A8N-SLI-SE
Hiper 520w PSU
Opteron 144
1 gb pc2700
1 gb pc3200
dvd-rw
Akasa all in one 5.25" panel
7900gt gainward 256mb
sweetx wireless adapter
win xp home
80 gb sata drive
160 gb sata drive

help! :(

what can cause a pc not to post?