THX is more a certification program than anything else. The re-EQ in surround processors is supposed to improve the correlation between the sound heard at home and the sound in properly calibrated theaters. Does it make a difference? Possibly. Not enough to make a buying decision on, IMO.
In order for a component to have the THX label, it has to be certified in several performance criteria, depending on the type of component. Just because a component doesn't have the THX label, doesn't mean it can't perform as well or better than one with the label. With that said, when it comes to receivers, it makes the manufacturer a bit more honest in the amplifier power ratings, by making the specification of power a bit more useful in the real world, like "100 watts/channel, two channels from 20-20K Hz"... rather than the usual garbage ratings of "100 watts, one channel driven into 1KHz" that plague the receiver market.
I don't think THX should be a sticking point. Just be careful to compare apples to apples before buying. Some "50 watt" receivers put out more power than other "100 watt" receivers. Look for a power rating that shows all channels driven, this will be a far better indicator of amplifier strengths and weaknesses than the one advertised.
