The hardware can be dissimilar. You tend to want to get the same servers, but this isn't a requirement (we have a 4/700 and a 4/900 clustered, for instance). As long as the lowest end server is capable of handling the load, that's what you're looking for.
K. Brian Kelley
http://www.truthsolutions.com/
Author: Start to Finish Guide to SQL Server Performance Monitoring
http://www.netimpress.com/shop/product.asp?ProductID=NI-SQL1
K. Brian Kelley
@kbriankelley