I've experienced this once. Went through pulling everything until eventually I removed the floppy and it booted. Put the floppy back and bingo, no boot. Something duff in the floppy or it's connections thinks I. Eventually traced it to a short in the floppy male power connector - fixed short, put floppy back and all well.
Try removing everything apart from the video card and a stick of RAM.
PS: You DID connect the aux (4-pin) power supply for the CPU, didn't you?