First, don't wait for the comps. All you need to do to get this started is to send a letter to the county appraisal district indicating that you disagree with their valuation of your home. You need to get this letter sent via some guaranteed delivery method before the end of this month, or no later than 30 days after you received your 2008 appraisal district notice, whichever is later. Either get the standard form from the appraisal district's online site, or just write a letter stating you as the legal owner, identify the parcel of land using the tax id (add in the legal description just to be safe), and simply state you disagree with their valuation.
Second, if you want to win your case, you only get 2 chances to do so, one at an informal hearing, and one at a formal hearing. Actually, there is a 3rd, but that requires you to sue the appraisal district. The appraisal district is much more prepared than you are, so get an expert on your side and go have a formal appraisal done by a licensed appraiser. It will cost you about $350 or so, but once you have it, if the appraisal finds your value to be less than the appraisal district, it's nearly a shoe in that they will accept the appraiser's findings.
And the benefit of winning this year will not only impact this year's taxes, but following years as well. I have seen many a client of ours try to do this themselves and fail. We always recommend they get a formal appraisal.
As a safety net to spending $350 and finding out that the appraiser thinks your home is worth the same or more than the appraisal district, go ahead and use some comp data from those that offered it. But if they are offering it, have them do a CMA for you. If the CMA shows you are likely lower than the appraisal districts value, then go get the appraisal.
Any realtor can pull up comps for your area. I would be happy to do so. Jeffrey's answer below is very good but there is an alternative to going to court with the appraisal district: binding arbitration. There is a form online that allows you to request arbitration through the State Comptroller's Office. You need comps, pictures, investor/owner information in your area, and $500 to register for arbitration. If you win, you get your value changed to what the arbitrator determines to be the market value based on the evidence produced by each side. Good luck!
Keller Williams Realty