The easy one first. The speed of the server in this specific application will have little impact on the throughput.
The problem with you proposed setup is that ethernet is a very inefficent interface protocol. Gigabit should provide 125MB/s (1000mb/8 bits in a byte), but in practice I have never seen more than 50MB/s, due to the limitation of network adaptors, switches, cable line noise, the mood of server gods, or whatever. The TCP/IP protocol is designed for reliability over long distances, not throughput. I believe it cuts transfer speeds in half any time it experiences an error, and in half again if the problem reoccurs. Other interfaces perform much closer to their specifications, like fibre channel, SCSI, or SATA. This is the same reason the Firewire400 is preferable USB even though USB is 20% faster on paper. In reality the overhead of certain protocols prevents anyone from actually experiencing maximum performance.
As far ar utilizing the equipment you had speced, a shared SAN does not pass the data through a server before it reaches the workstation. Certain SAN software solutions require a server to monitor array and direct traffic at the file level, but the data never passes through that server, so performance is not as much of in issue for a machine in that role. The MD1000 arrays you speced would work well as direct attached storage for your workstations since they are eSATA based, but Dell only supports them for server attachment. Even if they did, there is no way to easily share the data on them besides the slow option of ethernet. Similar arrays that interface with fibre channel would work better for you. Dell makes these fibre arrays as well, but once again limits their connection to PowerEdge Servers. I usually like Dell products, and am writing this on my Dell XPS 1210 laptop, next to my Dell Xeon workstation, but unfortuneately Dell does not offer any form of SAN product suitable for media creation or post-production. I am not too impressed with HP or IBM's offerings either, although I know IBM is working to improve that. The problem is that they are all used to the server mindset that doesn't work for post-production. Most of the good shared SAN solutions are provided by slightly smaller companies that are implementing the same basic technology in slightly different ways, to provide the workstation performance that post-prodution requires. What other industry needs 300MB/s to the average worker's desktop? Usually all of the high performance processing is done on servers that have the fast storage attached, and only the results are sent to the client system.
Your network engineer seems to have fallen into the same mindset, since he provided you a solution with extremely fast storage access from the server (2.4GB/s which is 24Gb/s) but only Gigabit connectivity to the clients (100MB/s which is 1Gb/s). So the server connection is 24 times faster, but you are only trying to serve 6 clients, so not matter what, at least 3/4 of your disk throughput (24-6= 18GB/s) is wasted.
As far as your other questions, yes that is my Qlogic switch, yes other vendor's solutions should be comparable. I agree $1K per seat license fee is a racket, but I believe the resulting functionality far outways the cost IF you have multiple users editing the same content.
As far as channels go, 4Gb Fibre theoretically delivers 500MB/s, Dual channel is 1GB/s, and Quad Channel is 2GB/s. 2Gb Fibre gives half the max performance at those settings. I would put the reality factor at 20-30% loss, so you should expect 350-400MB/s per channel PROVIDED that your disks support that rate. I estimate about 50MB/s per SATA drive. Also remember that you have two links to factor in. Each workstation has a dedicated link to the switch, but the connection between the array and the switch is shared by all the systems. Therefore you array should be connected with faster interface. In your case Quad channels to your array would be ideal for sharing to six single channel stations, but in reality, a dual channel link should be sufficient if you are doing compressed work.
If you are smart about the dual channel array you purchase now, it the future you should be able to purchase another identical one, stripe them together, and boom, you have quad channel solution, with twice the capacity, making use of all of your original investment, but now supporting uncompressed 2K to 4 or 5 stations at once.