Unfortunately it could be a number of things. Is the machine staying on, but you hear no beeps, no video signal, no nothing? If this is the case then it's generally your CPU, RAM, Or Motherboard, and you said you've tested your RAM, and replaced your motherboard. So currently I would think it's the CPU. Have you tried taking the CPU out and turning on the computer to see if you get any beep responses that way?
First thing I would simply try reseating everything..make sure theres no dust or a hair laying across anything as hairs can screw up your computer too. I had a hair laying across the RAM slot and when I put the RAM in it shorted two of the circuits and my computer would not boot, so stupid crap like that can have an effect. The best way to troubleshoot is to pull your motherboard completely out and put it on something non-conductive like a nice thick piece of cardboard that isn't on carpet or something. Then plug in the Video card, one stick of RAM, and the keyboard and see if you get any responses. You need to check every stick of RAM seperately to make 100% sure there isn't just one bunk stick..
Don't forget you'll still need to attach the PC Speaker that's generally inside your case to the motherboard to hear the beeps unless you have a PC Speaker that's attached to the motherboard.