RE: Intel vs. AMD
Unless you are into gaiming, a decent HTPC today probably won't need upgrades,
you can make it play HD/BD with under 50% CPU utilization and that's the hardest task for an HTPC today.
Also the 780G from Gigabyte has the 889A sound chip that does DTS encoding - I like it very much.
On a general note, I believe upgrades are overrated. I have used only mATX boards for over 3 years now,
and don't regret it a bit. Again, if you're a gamer (what I'm not) - it could be different.
RE: Vista 64
You can try it, it might work for you. I had nothing but trouble with it.
From drivers to codecs, filters, splitters, etc. But it's getting better.
RE: 4GB RAM
If you don't use a 64-bit OS, you won't see all the RAM. And for an HTPC 2GB is enough.
There is this issue of primary-secondary display and clone-extended setup.
Clone mode can only be used when the LCD and TV are the same resolution.
In extended mode, if you use overlay, video will be shown only on the primary display.
You have to use either the TV as primary (PITA) or VMR9 as video renderer (doesn't always work the way you want it).
There are no hard rules in building an HTPC: everybody has different ideas, different task assigned, etc.
Trying different configurations is part of the fun...