What is The Best Database for a Souped Up Hashtable?

All I really want is to store a very large Hashtable with more reads than writes. It should be able to deal with more than 8GB of data. The key will be a String of limited length. I want it to be extremely fast on read-access, stable with sudden shutdown and other issues beyond our control and with very low CPU usage. The leading contenders in my mind are:
1. HSQLDB
2. H2 Database, from the creator of Hypersonic SQL which is the predecessor of HSQLDB
3. Berkeley DB (Java version)
4. MySQL (least preference)
5. Derby Database
6. One$DB
7. Home-brewed implementation of a souped up Hashtable
8. Prevayler

Note: The databases are listed in no particular order.

Preferably it should leverage the memory for faster access using an LRU list for example. However it shouldn’t depend on being fully memory driven. To summarize high read performance with acceptable write performance and low CPU usage are my primary concerns.

Looking forward to your recommendations.