In all honesty its the best thing you can do. The problem with wanting HDMI 1.4 and a THX spec amp is your stuck with Onkyo and Pioneer. Yamaha, denon and marantz only get THX on there top spec amps and they are out of your budget. Pioneer has the same 'bright'sound as the onkyo's so you really need to ask for a demo as yu will be gutted if you spend all your money on an amp and then hate the sound.
What speakers are you using?? you will need to have neutral or laid back sounding speeakers to match the onkyo otherwise you could find the sound to sharp and fatiguing to listen too.
IMHO I dont think THX is as relevant as it once used to be.