So I had the opportunity recently to actually use up all ten slots in a clustered 7420 system. This actually uses 20 slots, or 22 if you count the clusteron card. I thought it was interesting enough to share here. This is at one of my clients here in southern California.

You can see the picture below. We have four SAS HBAs instead of the usual two. This is becuase we wanted to split up the back-end taffic for different workloads. We have a set of disk trays coming from two SAS cards for nothing but Exadata backups. Then, we have a different set of disk trays coming off of the other two SAS cards for non-Exadata workloads, such as regular user file storage. 
We have 2 Infiniband cards which allow us to do a full mesh directly into the back of the nearby, production Exadata, specifically for fast backups and restores over IB. You can see a 3rd IB card here, which is going to be connected to a non-production Exadata for slower backups and restores from it.
The 10Gig card is for client connectivity, allowing other, non-Exadata Oracle databases to make use of the many snapshots and clones that can now be created using the RMAN copies from the original production database coming off the Exadata. This allows for a good number of test and development Oracle databases to use these clones without effecting performance of the Exadata at all.
We also have a couple FC HBAs, both for NDMP backups to an Oracle/StorageTek tape library and also for FC clients to come in and use some storage on the 7420.

 Now, if you are adding more cards to your 7420, be aware of which cards you can place in which slots. See the bottom graphic just below the photo. 
Note that the slots are numbered 0-4 for the first 5 cards, then the "C" slots which is the dedicated Cluster card (called the Clustron), and then another 5 slots numbered 5-9.

Some rules for the slots:

• Slots 1 & 8 are automatically populated with the two default SAS cards. The only other slots you can add SAS cards to are 2 & 7.
• Slots 0 and 9 can only hold FC cards. Nothing else. So if you have four SAS cards, you are now down to only four more slots for your 10Gig and IB cards. Be sure not to waste one of these slots on a FC card, which can go into 0 or 9, instead. 
• If at all possible, slots should be populated in this order: 9, 0, 7, 2, 6, 3, 5, 4