RamNode offer a reasonable service on their 128MB/50GB SSD+Cache product. 128MB of RAM may not sound like much to you, and perhaps it isn't much.

Just think what computers used to do with such small amounts of RAM. They'd run an entire OS for starts. Windows 95 used to run within 16MB of RAM. If you're trying to cache gigabytes of data then you'll struggle within this environment, obviously. If you're sensible and you're serving content that is already processed and stored on disk, then you'll be ok since there's less processing to be done and most things will be indexed, again, if you're sensible.