I just went through this whole process myself, as I have 10's of light/appliance/outlet X10 devices, as well as the DS7000 system with many motion sensors and Door switches, and 4 X10 cameras with Ninja (robotic pan/tilt) devices. I also looked at most of the devices (Smarthome stuff, the ISY-99i) and other solutions, and had been rather disappointed with the MyHouse add-on as well as the high "cost" (power, etc.) of a PC. But after they introduced their new "Mobile" apps and moved the web server to your local PC (Active Phone, iWatchMobile Pro, OnAlert Mobile), I decided to go with a PC based solution. I found this unit at Newegg - an Atom system for $129 (
http://www.newegg.com/Product/Product.aspx?Item=N82E16856119025), added a hard drive, memory, DVD and USB hub all for under $300 and have a very nice low power system (normally runs under 20W per my UPS). I have an old keyboard/mouse/monitor attached, although I almost never use it - I use VNC for remote access and have the BIOS configured to ignore mouse/keyboard errors.
If you treat this system more like a hardware box (find an old copy of XP, add the free Microsoft Security Essentials, turn off automatic updates), then you won't need to worry about your stuff being obsoleted - if it works now it will keep working until you change something.
This was the only solution for my X10 cameras - nothing else works as well. If you don't have any cameras yet you can buy IP ready pan/tilt cameras for about $80 apiece that you can control directly, but I already had the X10 stuff and it all ties in together nicely. I also have the firecracker USB (CM19A) device and so I can support any of the X10 wireless devices as well. And sophisticated macros and "standalone" timers through the CM15A, as well as e-mails or phone calls via a USB voice modem.
Anyway, just my $.02.
Kevin