A little more specific hardware direction is in order, to reduce the hidden gotchas-
0) Run Cat5 cable to every place you might need it!
1) Asus M2NPV-VM mother board. The developers use it and know it works. I have had much pain using almost the same from others. They will work eventually, but its not worth the hassle.
2) 1 GB ram in the core/Hybrid. 500 MB in a media director. These work. More isn't used. (I have tested. . .)
3) The core will benefit from a dual core processor AMD 4300 or better.
4) A media director can work with a 3200 single core.
5) On board audio works. You can use an external AV processor for surround from the internal digital output.
6) Component out works.
7) Myth for live TV sucks- its very unstable (computer term for front end crashes or locks up). The back end works great and you can watch from the internal player. Use Mythweb from a PC to select the programs to record.
In the US if you can get OTA tv in digital from the channels you watch use the SiliconDust tuner. Its the best solution out there. Cable will be a murky mess that MAY work with the SD tuner or may not. Analog works with several tuner cards. Where I am (SF Bay Area) I get 27 digital channels from an antenna and the Cable tuner is unused. The OTA looks better and is free.
9) Use ZWave for lighting and other home control. The latest generation from Monster, Leviton and Cooper are all really good and will work with LMCE. X10 works some of the time but its slow and very flakey.
10) I will be posting on the WiKi what I did to get VOIP working in LMCE for both analog phones and a Nokia e70 cell phone.
11) Get a USBUIRT and a Windows MCE remote for each TV. its the most reliable and troublefree control. I like the gyro remotes but not everyone can use them easily.
12) The Panasonic security cameras are fine and cheap as they go. Not suitable for outdoor use as delivered.
Getting the above working right will take a month of tuning. Afterward I would look at things like irrigation controllers, security panels etc. Despite the pitch, while it may work it will need much tuning for your environment- lighting timers are worth hours of fun and anguish, VOIP is not accessible to humans- just geeks. And other pitfalls that will surface.
Many years ago Heathkit sold color TV's as a kit. It took 50-100 hours to assemble. You could buy an assembled working TV for less, BUT that gave the consumer no pride of accomplishment. Getting LMCE up, running and tuned to your liking will give the same feeling of accomplishment. And more capability than you can get from a $150,000 Crestron/Kaleidascape installation. (They don't even have HD or OTA TV integrated, let alone VOIP or some of the remote access/control capabilities.)