First remove that scsi card. Second remove some of that ram. There isn't a game in Mame that will run on a 400 that needs anywhere near that much ram (less ram means less ram for it to check).
Set your first boot device to hard drive 0, so it isn't checking your floppy, cdrom, and scsi first.
Finally, just let it run all the time, and simply turn the monitor off when you aren't using it. a 400 mhz computer has an ebay value of like $20, so it isn't a really big deal if you wear it out.
Btw, both my cabs take that long to boot too. It doesn't really bother me at all.