Minecraft wiki entry for Ramdisk_enabled_server, associated text: At least 4G of RAM on your machine is ideal
Enabling "Save Disk Image on Shutdown" impacts your Windows shutdown times where you leave RAMDisk running on shutdown, and similarly startup times for "Load Disk Image on Startup". This is not a problem if you manually start and stop the server only when needed.
Search the Internet for some ways to save your RAMDisk in case it does not work.
FAT16 is generally faster than FAT32 on RAM disks, however FAT16 formatting is not available for partitions over 2048MB
REMEMBER: Always have a backup! If your computer crashes, you lose any data on the RAM disk that has not been backed-up/copied to your hard drive!
Make sure that you allocate more than enough memory for the RAM disk than that of the size of your 'Minecraft Server' folder - remember that the Minecraft world data can increase by a lot!
...and on the other hand, don't leave too little RAM remaining for the running of Windows and the server itself.