How about Marantz? http://us.marantz.com/Products/172.asp
HD-DVD/Blu-Ray is using a new audio codec, either DTS-HD or Dolby-HD; be sure you undersatnd that NO receiver can decode the new codec at this time, and ask if your new receiver will be able to in the future ( firmware upgrade). At CES last week, Marantz was furthest behind the 8-ball technology wise. Also, your receiver must have HDMI input and output capabilities. Good luck.
You mentioned that you already have the Sunfire 5-channel amp. Do you really want an HT receiver (pre-amp/processor, plus 5-7 channels of amplification) or do you want an HT pre-amp/processor and plan to use the Sunfire for amplification?
If you are really looking for a single box solution (receiver) you might want to look at the Sony STR-DA7100ES. I own the DA5000ES which is a predecessor, and I really like it. I've owned Denon, Pioneer, B&K and other Sonys in the past, and the DA5000ES is easily my favorite. The new high-end Sonys use "digital switching Amplification" and they sound great. They are also very easy to use (IMHO).
If you are interested in the Sony, call Oade Brothers www.oade.com . They have excellent prices (better than any published web prices from my past experience) and they are authorized Sony ES resellers.(Disclaimer: not affiliated with Oade Bros., just a happy customer)
I would go with the recent Rotel HT receivers. Those with 1000 numbered models.
Integra Research is also worth considering, especially if you want multi-zone use. This is Onkyo's high end line.
THX is a George Lucas certification. If you think Mr. Lucas knows what sounds best, than yes you need it. If you don't care what Lucas thinks you don't need it.
THX is a great marketing ploy anyway.