Geothermal is very expensive.
Perhaps you can find an experienced BPI-certified energy auditor with an HVAC background who can recommend upgrades to your home's envelope and mechanical systems to provide cost-effective energy efficiency solutions with your current systems.
Make sure to look into all the available rebates, federal and local utility.
For more information:
Read Rick Goyette's Q&A "What do you think about geothermal heating and cooling systems?"
Also, read Mick Dalrymple's Q&A "Are geothermal heating/cooling systems better in reducing energy costs than solar power?"