I am switching over to WP-Super-Cache ( http://ocaoimh.ie/wp-super-cache/ ) and will remove WP-Cache ( http://mnm.uib.es/gallir/wp-cache-2/ ) with the release of Wordpress 2.4 on January 24. You need to disable WP-Cache before then. The step to properly switch are:
1. Go to WP-Cache page under options
2. Click disable it
3. Scroll down and click Delete Cache
4. Go to plugins page and deactivate WP-Cache
5. Login to ftp and go to wp-content under your sites folder
6. Delete Cache folder and the files advanced-caching.php and wp-cache-config.php
7. Active WP-Super-Cache on plugins page
8. Under options go to WP Super Cache page
9. Click WP Cache and Super Cache enabled then Update Status
If you need any help or have any issues let me know. Also with step 6 you might not have an advanced-caching.php file.