I think that historically, certainly some of the earlier clustering solutions required identical hardware on each node of a cluster.
When using the old Vinca Standby Server product for example, identical hardware was essential. It was even necessary to get the NICs on both nodes using the same IRQs.
This isn't the case with Microsoft clustering (or probably any of the other 3rd party clustering solutions available these days).
The nodes of an MS cluster can be of a completely different spec. Nothing even to stop you having an HP GL580 for one node and a Dell PowerEdge for the other.
Most folk do tend to use the same kit for both nodes given the choice though.
As mentioned previously, it's a good idea to at least try to have a similar amount of horsepower in both to ensure that in the event of a failover the second node is able to cope.