You should also become familiar with terms like SEER and EER. SEER stands for Seasonal Energy Efficiency Ratio, and EER stands for Energy Efficiency Ratio. There is an independent company that tests the systems under fixed conditions and applies the appropriate rating. Without getting too deep in the math behind SEER, it is a measurement of how efficiently a system cools over the course of the summer. So the higher the number the more efficient a system is. EER is a more direct measurement of how efficient a system operates when running at 100%, but less consideration should be given to this rating especially when looking at multi-stage equipment.
SEER and EER are important numbers and also determine utility rebates (utility companies like CPS and GVEC incentivize customers to upgrade their AC systems). The SEER range starts at 14 and can exceed 20 on some higher end systems. You should use your budget to maximize the efficiency rating and the number of stages, but be sure to have the duct work checked as well. Installing a 20 SEER variable speed system on a leaky, poorly designed duct system is like putting wooden wheels on a Ferrari.