are there any fans spinning? hard drive? does it display anything on boot? have you tried using each RAM stick in each DIMM slot?
did it work before the 3 years it sat? long periods of not-running is generally bad for components, especially hard drives
have you run it with just the PSU+board+CPU+graphics+RAM installed? is the power switch connector on the right pins? have you tried shorting the PWR SW pins with a screw driver to turn it on? (eliminating a bad switch as the problem)
are you using on-board graphics or discrete, and which connector is your monitor plugged into if you have both?
have you eliminated graphics as the problem? (have you tested the graphics card in another computer, or used a graphics card from a known-working system)
a few of the things that come to mind..
not-so-crappy 775 System:
Q8400 @ 3.8GHz 1.37v 24/7 - GA-EP45-UD3R v1.1 F12 - 4GB (2x2) OCZ Reaper HPC DDR2 1066 CL5 2.1v
EVGA GTX460 1GB 815/1950, w/ ACER 23" 1920x1080 TV & Dell UltraSharp 20" 1600x1200In Progress:
ThermalTake Armor A90 chassis function modded, 2x4GB GSKILL RipjawsX DDR3 2133 CL9, Corsair HX-750wRemaining:
Board / CPU / other 2x4GB CL9 ripjaws kit / GPU upgrade