It's not possible to boot from a PCMCIA device that size, and the transfer speeds over the PCMCIA port are really not all that great. I think a hard drive would be faster to boot from.
The ROM-resident C= carddisk.device only supports RAM (not Flash) cards up to 4MB. The excellent cfd.lha package on Aminet gives you a new device that can be used to read modern card formats, but it's a software-only DOSDriver, and isn't bootable. If you could somehow make the DOSDriver, associated .device, and filesystem reset-resident, then you could probably boot from it. That would require an initial bootup, however. Or the FlashROM of an Algor, Kickflash, or eFlash.
Another possibility is to create a boot floppy that mounts CF0: (the cfd.lha DOSDriver) and then reassigns the system to it to boot up. I did this before I had a hard drive in my A600. There is no speed benefit to this, however. But it is fun :-)
