Hmm I agree for a normal desktop mainboard, I wouldn't choose to use the Pico PSU. You may want to underclock your celeron to decrease power usage and probably run w/o a direct CPU fan if you don't need 1.2ghz of speed.
Web cams for color tracking: yes this can be pretty easily implemented depending on your knowledge of API for the OS you are using. In visual basic (its not a very good choice for algo processing) I could get about 2 fps at 320x240 @ 24bit rgb tracking changes in movement. Couple two webcams to your rig and you may be able to try out duo-vision algorithms such as those to measure distance.
Safeties: IMHO standard stuff such as stopping-on-loss-of-signal would be great. Your controller can monitor data from your PC and slow your bot to a stop when there is suddenly no data after lets say 0.5s, and your PC can stop your systems if there is a connection loss (depending what method of wireless comms you are using) after 1 second.
In addition you may want to try out low-latency audio/video streaming technologies too
