Just found a little program in the market (costs $.99) that actually seems to speed up my SD reads and writes on my Droid X by changing the cache size (can be set on the fly or at boot). I can really see a difference when loading music, photos and video files. The program does require root. So far I have not run into any glitches.
https://market.android.com/details?id=com.vaelek.sdspeedboost
https://market.android.com/details?id=com.vaelek.sdspeedboost