The Supermicro stuff is good. I recommend that.

Assuming you need to use the higher capacity (100s of GBs) 15k SAS drives for your current potential setup (12-port), I'd go with a 24-port SATA (Seagate ES2) system instead, since it will have about the same performance as 12 15k drives (perhaps more, since you have twice as much drives which also each have twice as much on-drive cache). Also, go with the Areca 1680ix-24 SAS/SATA controller. Areca controllers are generally less expensive than 3ware or adaptec, plus the 16 and 24 port ones let you put in your own DDR2 stick (though they come default with a ~256MB stick), up to 4GB of cache! So, for the same price as an equivalent 3ware controller, you get a controller with about the same (if not more) base performance plus 4GB of battery-backed cache.

Correct me if I'm wrong (please!), but if you're using autofailover, it's not recommended to use write-back cache on the iSCSI target (this refers system cache, NOT raid card cache), so having 4GB of raid cache vs. 512MB of raid cache really can make a big difference of performance when used in HA environments. Also, the Areca cards support RAID 60, which the 3ware ones don't.

Also, if you don't need 24 TB of storage, it may be possible to short-stroke the SATA drives to get some more performance out of them (anyone have any experience with short-stroking in the open-e environment?).