You can do this with some distributions, although all the ones I've heard of are based off a LiveCD (there are slight modifications to the kernel and some other configuration changes needed to make it work, and most are the same changes needed to make a LiveCD work).
Puppy is one example of a distribution that runs from RAM when installed to the HD (or any other configuration).
I believe Knoppix can actually be installed to an HD, and the toram boot option might work in that configuration as well.
There have been others that have done this, but it's generally uncommon, mostly because it limits the installed size of the system pretty severely (the entire system has to fit in a reasonable amount of RAM), and it's of limited value (most systems do a fairly good job of caching the disk to speed up the system without the limitations of an all-in-RAM solution).
I know there are tools to help you customize a Debian system; I suspect that these could be used to create your own personalized Debian system that runs entirely from RAM, as long as you're willing to limit the size of the system to a subset of your RAM (you still need room to run all those apps, figure 1/4 of your RAM for the ramdisk, the rest needed to actually run the system, but if you have 4-8G of RAM, then I say go for it).

==Joseph++

David Bendit wrote:
> I know all about that. Pretty much every LiveCD distro has that  
> capability. What I mean is, could a normal distribution, an installed  
> one, have the capability of loading itself into a RAM disk at boot?  
> It would be an interesting way to possibly increase performance in  
> times when data access speed is critical.
> 
> -David
> 
> On Nov 16, 2006, at 9:27 PM, Dennis Kibbe wrote:
> 
>>> On Thu, 2006-11-16 at 07:05 -0700, David Bendit wrote:
>>>> Actually, now I'm wondering: is it possible to have a standard
>>>> distro, say, Debian, load itself into a RAM drive on startup?
>>>>
>>>> -David
>>> Sure, you can run Knoppix that way.  Boot like this:
>>>
>>> knoppix toram
>>>
>>> It will take a while to copy the contents of the CD to ram.  Then pick
>>> up a 1Gb USB Flash drive for $25 or less and use that for your home
>>> directory.
>>>
>>> dennisk
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