In case you haven't heard, the Write SSDs the ZFSSA have been updated. Much faster now for the same price. Sweet.
The new write-flash SSDs have a new part number of 7105026 , so make sure you order the right ones. It's important to note that you MUST be on code level 2011.1.4.0 or higher to use these.
They have increased in IOPS from 6,000 to 11,000, and increased throughput from 200MB/s to 350MB/s.
Also, you can now add six SAS HBAs (up from 4) to the 7420, allowing one to have three SAS channels with 12 disk trays each, for a new total of 36 disk trays. With 3TB drives, that's 2.5 Petabytes. Is that enough for you?
Make sure you add new cards to the correct slots. I've talked about this before, but here is the handy-dandy matrix again so you don't have to go find it. Remember the rules: You can have 6 of any one kind of card (like six 10GigE cards), except IB which is still four max. You only really get 8 slots, since you have two SAS cards no matter what. If you want more than 12 disk trays, you need two more SAS cards, so think about expansion later, too. In fact, if you're going to have two different speeds of drives (in other words you want to mix 15K speed and 7,200 speed drives in the same system), I would highly recommend two different SAS channels. So I would want four SAS cards in that system, no matter how many trays you have.