XML, Yaml, database : before choosing, maybe you would like to have a 
look at Kirbybase : http://www.netpromi.com/files/kirbybaserubymanual.html
It has many very nice features :
- no installation (written in ruby) : put kirbybase.rb in 
c:\ruby\lib\ruby\site_ruby and that's all
- no database language to learn : everything is done with ruby syntax
- no administration tools required : the data is in plain text files
- no hundreds of page to read : the complete doc is 77k
- offers nearly the power of a "real" database system : indexes, lookup 
tables, one-to-many links, multi-fields sort, enough data types (String, 
Integer, Float, Boolean, Time, Date, DateTime, Memo, Blob, and YAML.)
- fast enough
- free
AF