I don't guess I see the need for multiple machines for a SAN though unless you just have a hojillion bytes of data. I don't know if I'd go with SATA drives for enterprise class, but for just data storage they would probably work ok. 3ware makes 4-port, 8-port and 12-port SATA Raid cards. They're not too expensive and neither are SATA drives.
With clustering, you're going to have a lot of overhead, both on the machines and the network. Most clustering setups will use something like Dolphin or Myrinet to connect the machines.
If you go the Dolphin route, you have to get an SCI Interconnect card for each machine, cables to go from them to a BxBAR SCI Switch and then you have to have the switch itself. This is just basic. There is a lot more to this setup.
If you choose the Myrinet route, you will need to buy the Myri-10G cards for each machine. These will support 10G Myrinet/10G Ethernet. You can also go with Myrinet Fibre Cards, but like the rest of the stuff mentioned, they start around $700 each. Then there's the cost of the switches, etc. Myrinet switches start around $4,000 and go up to ~$13,000.
If you want to look for a canned solution, Topspin is one of the best out there. They were recently acquired by Cisco Systems. The Topspin stuff can use Fibre Channel, Ethernet (1g or 10g) or Infiniband.
There is also the iSCSI protocol out there you might research and see if that would benefit your needs.
www.topspin.com - Topspin
www.myri.com - Myricom
www.dolphinics.com - Dolphin ICS
You might look here as well:
http://www.clusterworld.com/