SANs are generally built for expandability. You can add/swap hard drives and install additional hard drive enclosures if you have the money. The thing to look at is vendor, cost, and professional services. Dell, HP, etc are resellers for the hardware so it is easiest to go to them to get things started and provide pricing. If you don't know much about the SAN, it is very important that you get a professional services company which is prompt, reliable, and trustworthy. They will most likely install the SAN and support it until you get comfortable in managing the LUNs themselves.
As for vendors. EMC is a very trusted brand, but I hear Compellent is up and coming. Each vendor will vary in manageability and ease of use.
The best way to start is to contact your Dell or HP account rep. and ask them for a quote. They will be able to whiteboard the installation and provide additional services if you like.
If saving $$$ is a consideration, look into Novell. OES functions as an iSCSI target at no extra cost. Since OES is SuSE Linux, all storage is completely manageable, expandable and supported by Novell engineers. Managing the LUNs is a snap, too.