Welcome to the world of PC engineers.
OK this is what I would do if it was me... try and isolate components.
It helps if you have a spare system to borrow parts from.
1. Clear CMOS (check manual how to do that) - and disconnect ALL cables going into the motherboard apart from the POWER button cable, ATX Power, CPU power and the video card power cables.
2. Try to power on the motherboard outside of the case to ensure there is definitely no shorts anywhere.
3. Memory - use one stick, alternate sticks. Use different (known to be working) memory from a different system.
4. PSU - swap it with a different one.
... and carry on per part until it becomes clear which component or factor is causing the non-boot.
Definitely could be a fried motherboard or PSU problems though but don't rule ANYTHING out. Once built a system and the POWER button cable was causing a short!
Not bragging but worked for a e-tailer that sold a lot of PC systems, I have built hundreds, probably thousands of systems (and QA'd all systems we built) whilst I was there (4 underpaid years, overworked years.) Even now I still HATE it when a PC is built and it doesn't POST - however over 80% will POST after diagnosing the issue. Sometimes however it is terminal (a dead component).
EDIT: axeman got there just before me