If you're ok with only cooling the part of the house that's occupied you could install a split system for far less than replacing your main HVAC. I've been doing some research into the Mr Cool DIY units. I thought they were kind of gimmicky but they're actually pretty legit. Here's a 20 SEER for ~$2k.
It’s an interesting thought, but I’m not sure it’s really a good fit for my house. The house is a single story with a pretty open floor plan and is just under 1500 square feet. There’s not really a lot that could be divided off.