Of course you can use RAM. Its how any industrial pc runs a OS, or at least a good one.
http://www.linuxquestions.org/questions/linuxquestions-org-member-success-stories-23/how-to-boot-os-into-ram-for-speed-and-silence-662116/
Tho, not very easy to keep updated and secure and possibly will take it down several times before you get it running, and even then it will be a lot of maintenance.
There are Linux distros like Puppy Linux that load the system into a RAM drive that you can checkout.
As far as toarranre's reply, RAM Disks have been around a long long time. Going as far back as the PCMCIA SRAM cards... The hyperossystems drive, I have two ANS-9010b drives on my server, boots up perfectly and runs like a champ. Tho, in RAID 0 the speed is roughly 700MB/S continously, and 1.4GB/S burst with cache from my acera raid card (ARC-1210 4GB RAM). With access times around 9ms, would be a great database drive, tho I boot to it with Debian for my web server. System RAM is much faster then *MOST flash or RAM drives. I say most, cause of the Kove I mention later with link.
Also on both Linux and MS Windows, you can create a RAM drive.. On the Windows software versions you can format and install whatever you wish on it, but can't boot since it loads up with the OS. On linux you can script it to load it from your hard drive, and then use it. Which is what many people do to their MySQL databases, if you had enough RAM.
The OCZ RevoDrive shouldn't qualify or even be considered as a RAM disk tho, its flash..
But, if you wanna compared the TOP of the TOP, with RAM vs Flash, you should look at http://kove.com/xpress, which is the record keeper last I checked. The Kove Xpress should be the top RAM Drive compared to the OCZ RevoDrives... There is a obvious differece, and dollar price. Tho, both out of the reach of most people.