Use RAM Disk Mac OS X
In Mac OS X Snow Leopard 10.6, the 2.1GB limit on RAM disk size has been eliminated: any size RAM disk can be created, at least using the 64-bit kernel.
Just as important, 8GB memory modules are now available from OWC, which allows up to 64GB of memory to be used in an 8-core Mac Pro Nehalem, or 32GB in a quad-core model.
Speed of a RAM disk
In theory, a RAM disk should be ultra-fast.
Testing with DiskTester shows that in Mac OS X 10.6.2, a RAM disk can be written at ~1400MB/sec and read at about 2100MB/sec. That’s far faster than anything but the gnarliest RAID array, but is actually only a fraction of RAM speed. Still, it’s way more than fast enough, hardly any program can use even 1/4 that speed.
When a RAM disk is appropriate
You should also have at least 16GB of memory if you want to use a RAM disk.
A RAM disk is appropriate only for temporary working space (scratch)
How to Create and Use a RAM Disk – Mac OS X