Scale-Up means scaling to a more powerful server, e.g. from a dual-processor server to a 8-processor server or bigger.

Scale-Out means expanding to multiple servers rather than a single, bigger server.

You have to check the licensing and maintenance costs of a Scale-Out solution in comparsion to a Scale-Up solution. Do not forget the avaibility of your solution.

Here is a list of interesting articles, covering such problems with different RDBM’s

• Scaling out SQL Server 2005
• Achieving Massive Scalability with SQL Server
• Practical Guidance for Deploying Large Clusters
• Scale-up versus scale-out using Oracle 10g with HP StorageWorks Enterprise Virtual Array
• Testing RAC Scalability on DELL