but do they hold data when the machine is off
Yes, they have their own AC adapter.

The -per-MB solid-state drives do keep data when the machine loses power, which is probably why they cost so much since they use NVRAM or some such rather than something SDRAM.
Yea... hmm... I would be very willing to sacrifice that feature just to bring it into my price range... so I still think it must exist. I mean look at all the people willing to mess with RAID 0 setups to only achieve a slight increase in access speed. A ram drive like I hope exists would absolutely rape a RAID 0 configuration in all aspects, apart from size. And yea, it would take some messing around with, but just imagine, for 250$, you could have a GIG drive able to perform at the limitations of your PCI slot!!!