Only 2 comments on this one:
1. Watch the heat, especially if you trunk mount the box. I had a 233 MMX and had to deal w/ MAJOR heat issues. I live in Houston and the trunk on a summer day can hit 130 degrees. I switched out the system /w a 166 and added 2 muffin fans. Everything is well now.
2. NT is going to be a pig for the OS, not unless you are going to login every time you want to hear tunes. I would try to run Win95 OSR2 (for the FAT32 support) stripped down to the bone. I choose 95 because of the huge support of Winamp and plugins available. I have not had any problems so far. I have been running it for approx. 6 months w/out any glitches. From a cold boot to hearing music is less than 30 seconds.