Some of this is a little out of my experience level, so let me drop some thoughts for consideration(and I encourage other's input on this):
1-XEON's are potent cpu's and having 6-8 physical cores at around 3GHz and up should give very solid performance. I have a friend that bought an HP Z800 with dual quad-cores and he loves it. I recommend the dual 6-core as I use a lot of VI plugins that he does not. Whether you pick 8 cores is likely not an issue as long as you keep the speed up(around 3GHz as mentioned) and don't skimp on RAM(the mount of RAM in these dual-cpu systems is split between the 2 processors, so 24GB gives 12GB per cpu
2-you say you want to run HDX AND use lots of native plugins...what is your reason to "need" HDX? Consider that native plugins on HDX can actually exhibit more latency than the same plugin on an HD/Native system(HD Native card with digilink IO as opposed to an HDX card with the same IO). What is your end goal with this rig? Having worked on both, I have fewer latency issues(during tracking) on my HDN rig as compared to my friend's HDX rig that I also work on. I track full bands with a 64 buffer and several dozen plugins and no latency issues(unless I screw up
), but I choose tracking plugins for latency performance(my total ADC while tracking is 11 samples). My template session has all of my "mix" plugins in place, but those are inactive until I finish tracking(but I still run dozens of EQ's, compressors, Slate VMR, several reverb/delay plugins, etc).
3-you said you had no need to go past PT12 software? 2 things here; 1-remember that you need the HD version of the software(to run HDN or HDX card) and you should go to 12.4 as the minimum because that version will give you Track Freeze and Track Commit(2 features you really want)