HTPC Case, M/B, Processor recommendations

Hi everyone

At the moment I have a Shuttle SN41G2 v3 and it runs well, but I recently bought a 32" Sony Bravia LCD TV with a view to outputting video from the computer onto the screen... and hopefully HD video!

Unfortunately once I'd set the graphics card to the native res of the display (1360x768) even playing back basic xvid SD video in VLC was causing stutters, with the CPU percentage maxing out.

My AGP graphics card is fairly old too (I've had it a year and it's 2nd hand) NVidia GeForce 6200 256MB

I leave the Shuttle box on pretty much 24-7 apart from when I go away for any length of time, and I'm a bit concerned about the power consumption and heat of the now-aging AMD Athlon.

When the box is idle all it has running is uTorrent and possibly Firefox.

So I want a new motherboard/CPU that's very energy efficient when just idling... but with enough grunt to cope with outputting HD video at 1360x768, or any random video encoding tasks that I might throw at it.

If I want to be playing 1080p video, but obviously it would be downsized to 720 for the TV
Will that be more dependent on graphics card or CPU processing power?

I'm guessing a Micro ATX motherboard is likely to be more energy-efficient than a full ATX?
But the downside is reduced number of PCI slots with Micro ATX.

I need at least 4 PCI Express slots... dual DVB tuner card, graphics, and 2 taken up by my sound card (E-MU 1212M PCI)
Ideally I would have some spare ones... but if Micro ATV is the energy-efficient way, then so be it.

I was just wondering what my options are for motherboard/CPU and case, for a very energy-efficient system.
I guess Intel Core2 Duo is the way to go, unless there's some ultra-low power consumption Intel Core chipsets around?

I was looking at the OrigenAE cases, but some of the reviews question the build quality, and the price seems high.
I've also heard SilverStone as a brand but the products aren't displayed very clearly on their site so I can't really find what I'm looking for.

So really for this upgrade I'll need:
Case, PSU, motherboard, processor, memory (possibly can use my existing RAM), graphics card.

Any suggestions / ideas?

Thanks
Ben