limit your points of failure, remove everything but the video card, CPU and ram (might even want to try just one stick of ram, and try moving the slots around) Make sure to plug in all the extra power connectors, the 24pin ATX connector, 4 pin "P4" connector, as well as the 4 pin Molex and 3 pin floppy connectors to the motherboard. Reset the cmos and see what happens.
Also try this outside of the case, or triple check you have your motherboard standoffs in the right position. If the solder points on the underside of the motherboard are making contact with the case the computer wont post.