My personal opinion is your environment has a lot more growing to do before a SAN is necessary or worth the investment. You can get a couple of servers with 6-8 drives in them as Mastoo said for a lot less and end up with more storage capacity and a much less complicated infrastructure.
That being said, if you are going to be doing quite a bit of growing and adding servers (6+ in the near future, then the upfront investment may be worth while as you can recover some of the upfront expense of a SAN in buying diskless servers and attaching it to the SAN over time.
My suggestion, get a pair of servers with 8 x 450GB 15K SAS drives in them each. You will have 1.8TB of usable space on RAID10 available to you in each server and the 4 spindles will more than handle whatever you are throwing at them. Keep small environments simple.
The Dell MD3200i is good, make sure you have the ethernet infrastructure in place to support it as well. Moving your data through iSCSI works well, unless you tie it into a 100MB network that the rest of your clients reside on ;)
On another note:
If possible, try to keep your domain controller separate from your file server. It is generally best practice to do so. In my opinion, if your operation has you considering the merging of FS and AD, I would suggest your environment is not the right fit for a SAN either.
