I take it you tried using the old RAM again and it did not boot even with that?
If that's the case, my money is on the motherboard and not the CPU.
If you replaced the motherboard with the same model as the old one, or at least the same chipset model, you should be able to boot up just fine, with maybe having to setup some drivers. If you got a totally new model, you may have issues. Try booting into Safe Mode first and see. If you run into a big issue, the PC won't even recognize the drive as bootable.