I am thinking of setting up a Shuttle PC for mobile recording and as a HD playback unit for a projector. A few thoughts...
1) My 1st concern is the weight of the heat sink - this is probably the achille's heel of using a PC as a mobile rig. So, to minimize this I am going to use the latest 90nm AMD 64/3500. This chip uses less power than their previous chips. http://techreport.com/onearticle.x/7417
It looks like I might be able to throttle the power down a little too using the socket 939 "cool n quiet" technology, which scales the processor power use to the demand placed on the processor.
2) I am also going to use the older ATI 9600 video board b/c the newer ones generate more heat and use more power. Probably the SAPPHIRE ATI RADEON 9600PRO Video Card, 256MB DDR, 128-bit, DVI/TV Out, 8X AGP, Model "ATLANTIS RADEON 9600 PRO".
3) I will use the Western Digital Raptor 10,000 rpm SATA 36GB hard drive.
4) I am going to stick the 512 MB of memory that came with my G5 (I replaced the 2 sticks of 256 MB with 8 sticks of 512MB)...I think this will work as the socket 939 now allows for unregistered memory.
5) I wish there was a direct way to power this off of a battery (avoid the 110VAC transformer input).
6) I am still worried about the heat sink ripping the processor off of the main board if it is rattled too much.
7) I need another computer like I need a hole in my head.
8) I would think about going with Intel's latest board, but you can't use AGP, Intel processors produce more heat and are not as energy efficient, the Intel processors require larger heatsinks, the latest Intel socket seems to be pretty flimsy. It does have onboard sound (might come in handy with mobile playback), has onboard video and onboard wireless.