Beep codes on the D8400 come in 3 groups of beeps. So if you're hearing 2-2-2, that's a memory error. (Any beep code between 1-3-1 through 2-4-4 is a memory error.)
Remove all RAM modules except the one in slot 1. Clear CMOS by removing the motherboard battery and pressing the power button for ~30 sec. Reinstall the battery and see if it boots. Then swap all modules into slot 1 and repeat the process to see if you can find a bum module.
If they all work in slot 1, add one to slot 2 and try to boot. Keep adding modules to the other slots until all RAM is reinstalled or you find a bum memory slot.
Did you remove all PCI cards, except the video card? Got a spare video card you can try?
Do you know that the mouse and keyboard work? Can you test them on a different PC or try another mouse and keyboard on this system?