If you want to quickly make a RAM disk for use on your Mac, you could create one manually through the command line, or you can make it even easier by downloading TmpDisk for Mac OS X. The open source application uses a menu bar item to quickly make ramdisks from anywhere and of any size, the only restraint is that you have the physical memory to be able to accomodate the disks creation.
Throw the app into your /Applications/ folder and launch it to see the little disk icon in your menubar, click on that and choose “New TmpDisk”, specify a size, and a RAM disk is created instantly on the Mac desktop, ready for use.
RAM disks are temporary – hence the apps name – so you won’t want to place anything intended to be permanent there, but for cache files, photo and video editing, and anything else that you want absolute maximum speeds from, they can’t be beaten. Just remember to copy your files out of the temporary disk before you reboot or eject the drive so you don’t lose anything.