NoSQL databases are worthy of some evaluation, but they have a niche that they suit and it's not CMS' with 5,000 rows.
I think you should stick with a proper relational, SQL-based database. You may find PosgreSQL to be a better free choice than MySQL, but you'll have to evaluate this yourself1.
1 There's a variety of resources for this, for instance: http://www.wikivs.com/wiki/MySQL_vs_PostgreSQL.
