For an "regular" PC and monitor, you're not going to get below about an 80 watt draw. I built a good number of Athlon64 machines for one of our wings, and chose everything for low power. Between a 19" PC and the computer, draws from the wall average about 80 watts during use, with spikes up to 100 watts.
However, even "low-power" PC stuff isn't that great. I recently bought an Atom-based board, and despite the 4W draw of the CPU, the northbridge pushes the draw from the wall FOR THE MOTHERBOARD ONLY to 50-60 watts!
I also have a Via C3 system which, with a hard drive and optical drive, draws about 50 watts from the wall without a monitor.
My 12" laptop, under load, draws less than the 65 watts that the charger is capable of delivering, but at idle, still uses a little over 30 watts, thanks to the display - and that's a 12" laptop display, not a bright 15" or larger desktop LCD.
So, all in all, 30 watts probably isn't going to happen unless you get a REALLY good PC (or a laptop) and turn the brightness of your monitor most of the way down. You can get things like the Cherry PC or a FitPC, but then you still need to factor in a monitor.
