Probably bringing this thread back from the dead but I've been playing around with things this evening, namely I installed the free version of AMD RAMDisk (limited to 4Gb) since I recently upgraded to 16Gb in my system.
Since moving to my D800 my PC can tend to chug along when opening the files in LR, I can normally cope with smaller batches of images but it gets a bit tedious going through my larger sets waiting for images to load. I only have a quad core i5-750 (ancient by todays standards I know but serves me well otherwise) so I am still limited by that.
I didn't think it was possible to move the actual folders but searching online I found the following Lightroom help page which mentioned creating a symbolic link to the folder so Lightroom still thinks that the previews folder is kept with the catalog.
So far what I've done is create the RAMDisk and move the previews folders for each of my catalogs on to it (I create 1 per year for instance) as well as the cache folder. My catalogs and until now my previews and cache folders were all kept on my SSD for speed. I've then followed this help pages instructions and created symbolic links to the previews folders (the cache you can just specify in LR).
http://forums.adobe.com/message/3404204
So basically I have my previews for each year kept on the RAMDisk, I normally clear them out now and again as they get quite large so my folders may not be as large as some peoples.
My photos are kept on a single 2TB drive in my PC which is probably a limiting factor, LR still has to access these to create previews/cache etc.
In testing it is quicker, if you don't have previews for images already LR will still need to create them, this process seems quicker to me, I've just opened the Library module, set the thumbnails to their smallest size and wait for LR to create the previews. This is fairly CPU intensive but once done I can open the images a lot faster, the "Loading" notification clears a lot faster and I can zoom in 1:1 quicker on the first go too.
Again, on my PC I am on a lower spec than most people so your mileage may vary, I would assume newer CPUs will create previews much faster than my PC.
One word of warning if you are wanting to try this make sure you set the RAMDisk up correctly, there are settings to save the image to disk and save on exit etc, I've put mine on my SSD. Don't put files (eg catalogs) on it that you don't have backups for. I've limited myself to just the cache and previews as I won't mind losing them if something goes wrong. I could put catalogs on there but the free version is limited to 4Gb so they all wont fit even without the previews etc. If someone wants to try putting a catalog on there please post findings, I'd be interested to hear other peoples thoughts on this, I will probably try myself this week when I have more time.
!! I accept no liability for you losing data !!
RAMDisk (link to freeware version in top paragraph)
http://www.radeonmemory.com/software_downloads.php