... Depends on the size of your room
http/www.thx.com/products/home/logos/index.html?f=h2
As far as I know is that specific sound should reach your position at specific level without distortion, so a smaller room does not require a higher level since the sound could reach you sooner.
Games produces higher level of sound as they are all synthetically generated, unlike sound produce at studio where first generated from analog then digitally recorded so the sine wave is more natural.
Check Onkyo TX-NR901 or the Denon AVR-4308CI, while the Denon is not certified it is easily at the same power level as the Onkyo, If you want a Certified Denon you need to pay more for the extra certification.
With all this consider you still need speakers that'll handle your low distortion Amplifier so you'll need to spend more than $3000 for good quality speakers and hundreds of dollars more for the THX licensed technician to install the system for you just to get THX Certification. Some materials are also required to properly calibrate you Audio and Visual equipments. Example: room acoustics and design, treatments and isolation.
Are you ready to spent more than $20,000 for your THX Center?
My Entertainment Room is 14 X 16 with 8 1/2 feet of ceiling. If I have to get that place THX Certified I would Spend over $30K.