Yes, a fast card does make a big deal difference when saving RAW in CHDK.
I know, everybody keeps saying this, and now, I am just one of them.
My first CHDK test using the IXUS 140 where done using 2 different SD-cards. The first to use was an EMTEC 4GB class 4. The second card was a SanDisk 2GB ultra II. Both cards worked fine, even when taking handheld bracketed shots, but there was certainly some waiting whilst the images were written.
Today, I upgraded to a SanDisk 8GB Extreme, which is class 10 and, according to the manufacturer, can write at 60MB/s (marked with 80MB/s, which is the read bandwidth). Actually, I am not really sure about such speeds.
However, when copying CHDK, I noticed a severe difference in speed. Also when taking pictures as digital negatives (DNG), an increase in speed was clearly noticeable.
The 8GB card set me back by €23.-, which is essentially 20% of the camera's price, however, I believe it was worth the additional cost.