If you are going to replace the AC supply, the 250W unit should be fine.
However, the best idea is (and I know you know I'm going to say this) is the DC-DC. Arise has them for around $79 US.
Player: Pentium 166MMX, Amptron 598LMR MB w/onboard Sound, Video, LAN, 10.2 Gig Fujitsu Laptop HD, Arise 865 DC-DC Converter, Lexan Case, Custom Software w/Voice Interface, MS Access Based Playlists
Car: 1986 Mazda RX-7 Turbo (highly modded), 1978 RX-7 Beater (Dead, parting out), 2001 Honda Insight
"If one more body-kitted, cut-spring-lowered, farty-exhausted Civic revs on me at an intersection, I swear I'm going to get out of my car and cram their ridiculous double-decker aluminium wing firmly up their rump."
Where can I order the Arise? Is it AT?
You can order it on their website: www.arisecomputer.com
And yes, the PSU is AT