Hi again Jamie. I bought CoreCodec for my HTPC a year and a bit ago and it still serves me well. However, that was before ffdshow properly supported H.264 and before I bought PowerDVD Ultra with it's own H.264 codec. Personally i think CoreCodec is largely a solution to yesterday's problems - few H.264 codecs and barely enough horsepower to run them.
On my office system, almost the same as yours but running the Q6600@3.4Ghz I am quite happy using the latest stable ffdshow tryout for H.264. If I had the Cyberlink codecs on this system they would probably be fine too. I am not considering paying for CoreCodec for this system.
Regarding the rest of your software list. For pure HD the list above is a good start point. As you know I use Meedio with Playmee to auto mount the isos before launching PowerDVD. Not sure xLobby can do that.
I would add Powerstrip and Reclock, so you can use Reclock scripts to cause PowerDVD to change the refresh rate to match the media you are playing - 24/48/72Hz for all HD (even UK HD/blu discs), 50Hz for PAL DVD, 60Hz for US video. However, Reclock certainly does not support EVR and I do not know if it works even using VMR on Vista. Also, Powerstrip cannot change timings with the nvidia 8xxx series. Reclock is one reason why I have no intention of moving to Vista, the other being I have Vista on my office PC and in my experience the whole platform (OS/drivers etc..) is still no way stable enough for HTPC use. It is also the reason why my next graphics card (I currently have the 7600GT) will be an ATI 3850. However, I guess if you really are only going to play HD you may be able to lock your refresh rate at 24/48/72Hz and be done with it. However, you will still have problems for a while eg. with PAL DVD extras discs with HD DVDs which will need a 50hz refresh rate for smooth playback!