Riak 0.14 is out
github.com
github.com
http://wiki.apache.org/cassandra/CassandraHardware
I can't find any numbers for riak RAM requirements...
If you observed cassandra consumes a lot of RAM even with small dataset, you should tune the GC settings. Besides it, JVM itself consumes a lot of RAM.
Cassandra does scale down (I know at least one site that went live running Cassandra on a 256MB VM) but that's mostly of academic interest to the audience looking for a scalable database. Hence the recommendations on that page.