However, my advice to you is, after researching your area to determine an appeal for reduction, to also take photos of any bad parts of your house (perhaps chipping paint, foundation cracks, etc.) and take them with you to your appeal hearing as this makes a world of difference in having your appeal granted.
Hope this helps!
You can search Dallas County properties at http://www.dcad.org for the tax appraisal values. A local Realtor should be able to get you a more accurate Market Analysis to help your case with actual "market values".