I tried to use a a PCMCIA flash drive as a scratch disk in order to boost performance on my Lombard before it got nicked. Didn't work since on the Lombard at least, the scratch disk initialises before the PCMCIA drivers. So no joy there.
I wanted to boot from it, but despite my best efforts, I failed to strip Tiger (or it may have been Panther) down to under the 1GB size of the card. I suspect it wouldn't have worked anyway, for the same reason as the scratch disk.
CF to IDE or SATA should work fine. SATA will probably give a better speed boost over the HD. RAIDing two of them is only going to help if you have two independent busses for them. You can do this with a PowerBook, but you will lose your optical drive. RAIDing two with an iBook will also lose your optical drive, but won't be any faster than a single card anyway, since both drives are on one bus.
MacBooks and MacBook Pros have only one SATA connector, and RAIDing across SATA and ATA is silly.