You have two choices.
1- Boot into safe mode via CD rom. There are a number of Linux based "Mini" os;es out there you can burn to a thumbstick and boot from that.
2- Jump your Bios and reset it. Keep in mind, if you do it wrong that board is going to become a brick.
3- Take off the parts you installed for now, try to boot again. Then when it settles down, install one part at a time rebooting each time.
If your Bios still wont accept it, then you will either have to "Flash" it or replace the board with something that already contains firewire and has enough dimms for the memory you have.