I have a m9000t, which is the model that was out 9 months ago. Here are my experiences.
skf:1. Can you upgrade the O/S to the retail version of Vista Ultimate? (using a key for Vista Ultimate)
I upgraded from HP Home Premium to Ultimate with a retail upgrade. (You will need the DVD and an unused key). I ended up completely formating my OS drive and doing a clean install. HP includes alot of software I did not want.
skf:2. Can you replace the video card with any HDCP compliant video card? (like the one I currently have in my Vista Ultimate VMC box)?
I have not done it.
skf:3. What do you need to do to activate the external Cable Tuner?
If you buy a m9300t with a tuner, the PC will come with an external digital cable tuner and a digital cable product key (in addition to the windows product key). If you use the vista install that comes preloaded you will never need to do anything with the digital cable product key. Even if you restore you OS from HP's restore DVD or restore partition. HP doesn't require you to activate your digital cable product key. If you install vista from a retail DVD you will need to type in the product key during media center setup to activate the digital tuner.
skf:4. I can simply add an extra external ATI Cable Tuner to get two, yes?
YES, I have been using two.
skf:5. Are there any difficulties I should be aware of in hookup up VMC with a Digital Cable Tuner (things that don't work)
Firewall software (norton) will block the digital tuner. The tuners operate as a network device, you need to turn the firwall off for the tuners IP address. When my service provided initially came to install the cable cards we could not get the tuners to recognize them. Ended up that both cable cards were bad. New cards were brought and everything has been working since. One final issue I have experienced is that when I setup the PC to sleep, once in a while I would get a "No TV Signal Error." With the PC on 24/7 I never get the error.
skf:6. I can transplant the motherboard etc. into my really nice Zalman HD135, yes?
YES, HP uses an ATX case.