Hardware specs

[Mark]

Can someone give some brief guidance on some (ballpark) specs for a 
hadoop cluster?

- What are the typical size and memory requirements are there for a 
dedicated namenode?
- What are the typical size and memory requirements are there for 
individual datanodes?
- Should there be more, less or even amounts of ram on each datanode?
- Should the namenode have more, less or an even amount of ram as 
compared to the datanode?

Thanks