SQL Cluster 2008 need not the exact same hardware but rather compatible hardware. I'm currently running a 2 node cluster with Node A (64proc, 256GB) the primary and Node B (24proc, 128GB) as the passive. Now where this becomes a problem is if Node A crashes can Node B handle the load? In our environment it can but queries do become slower and its okay for us because it is only temporary until we get Node A back up.

It all depends on your load and can the weakest link (Node in this case) hold all your resources?

-king