If you have gas appliances OTHER than the furnace (like you stated with your dryer, how about the oven?), look at the gas usage delta between your coldest months and the warm months. This will give you an estimate of how many therms you are using to heat. Now, multiply this usage by you furnace efficiency (probably around 60% based on age and technology from 24 years ago), this will give the amount of gas turned to heat. Divide the amount of gas turned to heat by the efficiency of the new system and you now know your cold month usage. Scale that # for all of your heating months (or, if available, ratio of the total heating days/year to the average heating days from your original "coldest two months" of the equation) to get your new gas usage for the year. Multiply by the cost of a therm of gas and you now have your annual gas savings in $$$. Lastly, do the net present value of those annual payments to determine breakeven point.

Rinse and repeat for electricity usage, but use the hot minus cold months KWH average. This one is a little trickier as the electricity usage of other major appliances also go up in the summer because you probably don't keep your house at the same temp year round. Also, you furnace blower with have some different inefficiencies with the new setup.

OR, you could simply wait until one of them breaks and replace them both then since it is unlikely the new units will ROI before the units fail. :)