Been watching this for a while. It's a cheap (current prototypes $150, should be $50 after a while) PCI card that holds up to 4 gigs worth of old, slower DDR-200 and hooks up to a SATA connection. Yup, it's a hardware ramdisk.
When I first saw this, my general feeling was that, even with the built in battery backup, it's not really safe to store anything that you can't afford to lose on a ramdisk. So, I would never recommend you install your main Windows partition on it. (As it turns out, doing so doesn't cut boot time much anyway. They went from 14 second boot to 9 second boot when they tried this. Most computers will take much longer than that just to POST the RAM.)
That said, however, there's a lot of stuff that you CAN afford to lose. Almost any game you buy will be re-installable at your whim from the original DVD. So why not throw it on there? Similarly, the Windows swap file. It gets wiped out every time you reboot. How does a 4 gig swap partition only a few nanoseconds slower than main RAM sound?
So the Anandtech guys went and tried it out. And, highly disappointingly, it turns out that there's very little real world benefit to it. Games on the ramdisk only load about 20% faster. I suspect the real bottleneck there is that level geometry and textures are stored compressed, and so decompressing them is a CPU-bound operation. Also, a lot of games require more than 4 gigs of disk space to install, which makes them too big to fit on the ramdisk. (And Gigabyte says not to RAID 0 multiple drives, because there are issues with that.)
The swap file thing works fine, but it turns out you'd be better off taking the money you spent on the iRAM and using it to buy regular RAM instead. The only real advantage on this one is if your OS manufacturer is a short-sighted idiot and you're limited to 4 gigs of RAM, this will give you another 4 gig that's almost as fast, but doesn't run afoul of the 4 gig RAM limit. Though you'll still be hostage to your OS's ability (or lack thereof) to swap efficiently.
The real place where this thing shines is in the complete lack of seek time. Access one byte at the first sector, then access another at the last. No latency. It would be a great scratch partition for people editing HD movies or other HD video that won't fit into main RAM. Another good use they mention would be a media PC. Since this thing is silent and seeks are instantaneous, it makes for an awe inspiring MythTV box. (Though that 4 gig limit looks awfully annoying here, too.)
I was planning to put one of these in my next machine, as fast swap. However, I recently decided that I'm not buying a new machine until the 4 gig RAM limit is resolved. Seriously, you brainless dorks in the PC OS industry have no excuse. I'm not that old and I can already remember this happening twice. Once going from 16 bit to 32 bit, and now from 32 to 64. Where are you morans going to figure this out? THERE IS NO SUCH THING AS ENOUGH RAM. Never has been, never will be. Quit designing OSes that assume there will never be more than X bits on the address bus. History has proven you wrong twice already. When are you idiots going to learn?