You have ddr2 and ddr3 in the same tower? Or you had ddr2 in it and changed it to ddr3? If you're using both, try removing the ddr2 and booting with just the ddr3. Try vice-versa if that doesn't fix it. Even one bad card will screw the whole thing up, so you might have to try removing one, booting, remove a different one, boot again, etc....