SSD Cache

Yes, you can (assuming that the cached drive is a conventional 'spinner'). But you will probably not be able to remove the contents of the cache (unless the motherboard software allows it; I don't think that mine (Asus) does this).

To be honest, any form of caching is a security nightmare. I use SSDs as the system drives in both my desktop machines, and am very careful to store sensitive data on non-system drives, from which it can be more reliably erased. For this approach, I find that 120GB SSDs are quite adequate, and the price difference between those and the (maximum) 60GB drive you would use for caching is now quite small. That is why I have not used the caching facility on my newest motherboard.

While we are talking about SSDs, it is also worth repeating the basic advice that file and folder erasing is defeated by the wear levelling mechanism on the SSD, and so does not work, but that free space erasing, which fills the drive, does work.

You may want to also disable write-back caching on your mobo BIOS before you do an unused space erase... otherwise you'd have wasted many, many program/erase cycles on your SSD...

But yes, there's no way to securely erase the data on the SSD, so sensitive information could be lurking anywhere in the SSD. Remember also that SSDs have larger "physical sector sizes" than do HDDs, so larger contiguous chunks of information can be gathered from a SSD than from a HDD, though wear levelling does complicate recovery from anything longer than one "sector."

I quote my "sectors" because your motherboard still sees traditional sector sizes (512bytes) but in reality the SSD's page size is usually larger (4K or more)
Joel's point had not occurred to me, but it makes sense. As I understand it, the erasure will find its way to the target disk, but only (or often) via the SSD cache, with the consequent increase in wear that Joel describes. This reinforces my doubts about the caching solution as a means of getting SSD performance at low cost.