remove the MBD and check power connections. Make sure nothing in there is grounding to the case (IE connectors, screws, etc... ). When installing a MBD this is a possible cause, check and see. Also, try to reseat the mem and processor. Careful with ESD!!!!! If that doesn't work, then disconnect everything except the mem and processor and boot up. NO you wont get a screen, but maybe at least a beep code. Then hook everything up ONE by ONE! HDD first, then Vid card, then CD, then Floppy, etc.
OH, and also when you started piecing them together... Did you take the best parts of 2 computers and put them in one case??? So one nice system and one cheap-o? If so, then the power consumption maybe to much. What are the specs? This is for the good one, by the way... the other can be dealt with after this.