A cracked heat exchanger is potentially hazardous to your health. As jumptrout said, the new ones are safer and use less gas. I really like the Rheem/Ruud RGPS series, a solid furnace priced as a builder's model. If you would like a little upgrade go with the RGPE series; this is a 2-stage furnace with a variable speed ECM blower motor. For about a $600 premium you are planning for the future as pretty much any A/C unit over 14 or 15 SEER requires a variable speed indoor blower. This furnace also has serial communication capabilities. The 2-stage heating is really nice and makes the home more cozy and even temperatured. Since these new furnaces may last 30 or 40 years you may want the upgrade if the budget allows. The RGPE model is compatable with all the Rheem/Ruud A/C models from the entry level to the top of the line 2-stage cooling units. If you have central A/C now that variable speed blower can increase your existing SEER rating by about a full SEER point. Don't be at all surprised that the new models will have a lower BTU rating than your old dinosaur. HVAC engineers have learned over the years that we don't need as many BTU's as once thought. In addition, the higher AFUE models waste much less heat and burn very cleanly.
In Atlanta you have lots of competition amongst contractors so once you determine the size you need to get multiple estimates. You may find as much as $1000 difference between highest and lowest prices. Look in the yellow pages and find the local Rheem or Ruud wholesale distributor in your area. Call and ask for someone at the A/C counter. Tell them you want to have a new furnace installed and could they recommend a couple of honest dealers to do the work. Be sure and get the name of the person providing the references. Have one or two dealers look at your old furnace to make sure there's nothing unusually difficult with your change-out. If there is they will certainly mention it. You can then get pricing over the phone. Once you choose a dealer have them stop by and look at your setup (if they haven't already seen it) before the day of the job so there are no surprises. Get a written estimate before the work begins. A reputable dealer won't ask for any money up front but will expect payment in full once the work is finished and the new furnace is fully tested.
I'll add that the Rheem/Ruud equipment has been at or near the top of Consumer Reports' dependability ratings for several years now. You will find that most contractors are partial to the brand they primarily sell so choose the brand first. A good question to ask is who is the wholesale distributor in the area and how many branches do they have. Many furnace parts are OEM only so it's important that these parts are stocked locally. The more brances, the bigger inventory.
Source: Fraser-Johnston Furnace repair possible?