When to use a RAM disk?

I am not sure about using a RAM disk for speed advantages, but they can be used to create a secure area. If you create a RAM disk and use it as a working area, any temporary/sensitive etc files created on it will go as soon as the machine is turned off.

RAM disks can slow down your system.

The cache gives you a lot of the advantages of a RAM disk. The more memory you allocate to other things (such as a RAM disk), the less that's available for the cache. And the cache speeds up pretty much everything that involves the hard disk on your PC, not just the stuff you put on the RAM disk.

On the other side, there's virtual memory. The more RAM you've reserved, the more likely your applications are to be repeatedly thrown out to disk.

This is one of the reasons why RAM disks pretty much died. Back in the days when people had maybe 1MB RAM, RAM disks were quite common (though more so on Ataris and Amigas than on PCs). Now they're rare, even though Microsoft has offered a RAMDisk (for free, IIRC) for a long time. This seems absurd, but that memory is much more of a carefully managed shared resource than it was back then. When people had 1MB RAM, you'd have been laughed at for suggesting that buying memory would speed up a PC - either the app would run in the space available or it wouldn't. Then, Windows 3 happened, and things changed.

That said, a RAM disk can be an advantage in special cases. Just make sure it's a genuine advantage, and not just you sabotaging the efforts of your O/S to run things faster.

EDIT Having said all that, I really should answer the question ;-)

imdisk is a good virtual disk driver, and the virtual disk can (optionally) live in system RAM. The installer supports both 32-bit and 64-bit Windows. It's supported on Server 2008, but check the notes as there are UAC issues.

In some cases what everyone said is correct. In fact, it hasn't died out. It died out for a period, but it was brought back. The best combination you can do is the RAM disk and an SSD.

Most programs now have the ability to save on shutdown, so it migrates the data from the RAM disk to your SSD so none of your data is lost. To whoever said they don't know if it has speed advantages, the read/write speeds are up to 70 (more or less) times more than an SSD, but like you said, the problem of data deletion occurs.

There are somewhat large differences in speeds depending on what you're using the RAM disk for. Anything that requires an intensive read and write rate then RAM disk outperforms anything. Compilations, coding, data intense games (Battlefield 3 maps, Crysis, etc.), and stuff like that would benefit greatly from RAM disk vs. an SSD.

For example, a Minecraft Tekkit server is extremely data intensive. It's always generating areas, multiple directions, and in multiple parts of the map, always generating machinery, user data, plugin data, data logs, machinery energy, alchemist energy (extremely intensive, it gets so bad that server owners have to completely disable the mod), etc. Minecraft is infinitely generating the map and that will require A LOT of read/write speed.

RAMDisk is the best option for that. Pair it with an SSD for when you shutdown the computer/server, and it'll practically be instantaneous. Any application like that would benefit dramatically with RAM disk, so no, RAM disk isn't obsolete; it's being brought back if anything.