Not to burst your bubble here, but your last statements are off the mark. You can't just buy equipment that is THX certified (technically this goes through the THX Multimedia Certification process, which is different than Theatre certification), and claim that your home theatre is "THX Certified".
THX Certification of a theatre (even a home theatre) is a process that must be done by approved technicians, making specific measurements that take into account many variables in your building.
These include: The Physical Structure, Projection system, Seating Arrangement, and Sound System.
For example, your equipment would have to meet certain standards as built, but it would also have to meet standards in use, validated by measurements. In other words, you'd have to pay for it...
For a brief discussion of this see:http://electronics.howstuffworks.com/thx3.htm
For the Actual certification details, see here:http://www.thx.com/mod/services/homeCertOview.html
Finally, your statement about the value of this certification and the potential for increasing the value of your home on resale is highly subjective at best. There are many threads that have discussed the contribution of a home theatre to a home's value on these forums, but I would say at the very least these are statements that are not easily proven.
Some people indicate that the home theatre is a turn off for some buyers, while others think it is a plus. I would say that the incremental addition of THX certification of that home theatre does little but limit the number of buyers interested in looking at your home; you'll likely price yourself out of the market. However, if you can find the one person in a given market willing to pay your price, then you've succeeded. Just a reality check...
Good luck with your build!
Edit - This information on the Home Theatre certification indicates that the designer/installer would themselves have to meet certain requirements, including a CV showing a history of project or membership in Cedia, for example, to qualify.http://www.thx.com/mod/services/CHTT...alOverview.pdfTo me, this means most of us DIYer's here at AVS would not be able to get our home theatres "THX Certified"...