First, try taking everything out except the cpu, fans, and ram. If the green light is on and nothing goes, It's either a bad PSU, or you didn't connect the power switch cable properly on the mobo. Honestly, quadruple check that cable, I think most mobo's have the pwr switch go on pins 2 and 4? I've accidently put it in the wrong spot many times and run into this problem.
Current Build (Hope to finish by late spring):
Intel Q6600 g0
Asus Maximus Formula
geIL Evo 2GB (2x1gb) DDR2
ATI HD3870 X2 or Nvidia 9800 X2?
2x 150gb RaptorX (Have it)
Thermaltake Toughpower 750w PSU (Have it)
Antec P182 Case
HP w2207 22" Monitor (Have it)