I would never use the CMS collector with less than 1GB of heap space. And I would not use the CMS collector unless I understood what effect it had on my application and system in general. Never ever ever use the CMS collector on a single CPU system. I would not turn it on unless I had at least 4 CPUs.
Do not rely on any undocumented options - their availability and behavior change from version to version. Stick to the options available here:
http://java.sun.com/javase/technologies/hotspot/vmoptions.jsp unless you want to get the source for the JVM you are using and looking up the current usage. For example, in JVM 6 the -XX:+CMSPermGenSweepingEnabled option results in a warning to use -XX:+CMSClassUnloadingEnabled instead.
Also, the statement that the GC ignores the permgen is not entirely true - if you turn on one of the GC monitoring options (PrintHeapAtGC, I think), you will often see the collector releasing classes from the permgen. OK, in the source I see this comment:
So turning on the CMS collector turns off permgen cleaning unless you turn that cleaning back on.