With the move to CS5, many Adobe apps require a 64-bit operating system. What this means is more speed and a greater ceiling for memory. It also means you'll need to update your plugins.
Here's a definitive list on the state of After Effects plugins right from Adobe.
After Effects CS5 requires 64-bit plug-ins as well as a 64-bit operating system. As plug-in developers release 64-bit versions of their plug-ins, we will list them here. Most plug-in developers are working on 64-bit versions of their tools, and many have announced that their plug-ins will be available at or near the time that After Effects CS5 is available. Check this page often for updates.