I have a Via M10000 Mini-ITX motherboard in my carputer. 1Ghz, 512MB RAM, etc etc etc.
Excellent motherboard, super easy to setup, and works like a charm. Are you SURE you need 1.5ghz? Seriously, unless you're playing Unreal tournament or something, 1ghz-ish should be fine. These motherboards are super low power consumption setups, you're screwing yourself if you sacrifice all that battery life for an extra 5% of performance.
In my carputer, I have 3D GPS, MP3s, OBD-II logging and more running all at the same time in the background, no problem. When I'm parked, I can even play DivX encoded movies perfectly. I'm very skeptical that you need that extra processing power, but check out Via's offerings, they've gotten a little faster since I built my setup.
Another advantage, you can get inexpensive DC-DC power adapters for these motherboards since your robot will have a steady power source. My carputer uses a spendy Opus 90 watt supply because it runs a lot of peripherals and has to live with the dirty power that a car outputs.