None of the current express card SSDs that I have seen have speeds that are superior to USB2, so they are not particularly suitable for a scratch disk on laptop. I think the best bet is to get an express card eSATA card and an eSATA hard drive.
Wrong, a few SSD expresscards don't use the USB interface and are much faster, the wintec filemate 48GB is one of those.
You can find an article on it here : http://www.macworld.com/article/145185/200...mbpssdcard.html
I have it myself (I use it as a boot drive), and I'm quite happy about it. Since it appears as an additional SATA drive, there is no reason for CS4 not to accept it as scratch.
Here is my review (in French, but you have speed tests, which are easy to understand) : http://www.macbidouille.com/articles/316/page2