You can keep a image of the boot disk (ISO for example) on your hard drive. When your computer boots it would have to run a command that creates a "ram disk" in ram and copies the image from the hard drive to the ram disk.You would need about 16GB to 32GB of ram to do this, 32GB for best results. It is definitely possible, try a Linux bootloader which is most likely to have the capability. The problem would be separating the OS files from installed applications (if you have 20GB of apps it would not work well with those on the ram disk) and being able to switch between Hard drive (apps and data) and ram (os system files). The operating system might need adjustments so it still thinks all files are in the "same place". If you have a lot of RAM the operating system will take advantage of it anyway and you are already benefiting from the significantly higher speed of ram. You can create a ramdisk and just copy files there for fast access (like an application or a video your editing, or a library of images, etc.)
In the Terminal app (built in on all macs), use this command:
diskutil erasevolume HFS+ "ramdisk" `hdiutil attach -nomount ram://9800000`
This command will create a virtual hard drive in memory (RAM) and you can copy files to it. 9800000 represents a 5GB drive. If you have less than 16GB of memory I would reduce this number by half, for a 2.5GB drive. Any file placed here will be deleted when you reboot or lose power, so copy files back to your hard drive often to prevent data loss. When you open a file from the ramdisk it will be on average about 30 times faster than from a hard drive.
