It probably does not matter whether you signed the ticket or not. Please confirm this advice by contacting a speeding ticket lawyer in Atlanta (I have provided a link below to those offering a free consultation).
Yes its lilely valid. Further, If you don't appear for court, nobody is going to take the time and say, "hey that guy didn't sign the ticket, we should dismiss the case." Instead, the judge will probably order your license suspended for failure to appear. The judge also retains the power to issue a warrant for your arrest for failure to appear, however, that is rare.
Your signature is merely your acknowledgment that you received the ticket. You likely did sign it but the imprint of your signature didn't go through. If you did not sign it, the officer would probably have told you that your refusal to sign it authorizes him to take you to jail so that you would be booked and be forced to post bond guaranteeing your appearance in court.