Workload analysis before migration
If it's SQL Server 2005 or later, I'd initially look at file stats, which shows you how much disc IO each database uses, together with memory stats:

Memory Breakdown:

SELECT
   (CASE WHEN ([is_modified] = 1) THEN 'Dirty' ELSE 'Clean' END) AS 'Page State',
   (CASE WHEN ([database_id] = 32767) THEN 'Resource Database' ELSE DB_NAME (database_id) END) AS 'Database Name',
   COUNT (*) AS 'Page Count'
FROM sys.dm_os_buffer_descriptors
   GROUP BY [database_id], [is_modified]
   ORDER BY [database_id], [is_modified];
GO


Microsoft provide an Assessment and Planning Toolkit which you can point to your SQL Servers, and it will poll every 5 minutes to gather useful information such as IOPS.

For a consolidation/migration project, Bob Duffy's (MCM) SQL Bits presentation video is full of tips.