I think you can divide this into capacity - performance - connectivity
First of all, you will need to know what your current used capacity is of the systems you want to migrate to the san.
Next, what is the expected grow of the data? Are there new projects that will need to be deployed onto the san ?
With regards to performance, have a good look at your applications, and determine if it is reponse time or lot's of IO's that are done. You vendor of choice will need to know this information to determine the correct type of controllers and the needed amount of spindles required to obtain the performance you need.
With regards to connectivity, you will need at least 1 HBA with 2 ports to connect to your san ( for redundancy ). If the san is a clustered solution, you will need 2 x as many fc ports. A good practice is to stuff 2 dual port HBA's in each server connecting to the san. Generally speaking, be sure to have at least a dual fabric in place if you are looking towards a high available solution, so expect some costs in switches,fibers that need to be put into place between data centers ( if not already present )
Now you know what you need, contact your favored vendors ( IBM,NetApp,EMC,HP,....) and ask their presales engineers to make a correct sizing
Hope this helps
rgds,
R.
NetApp Certified NCDA/NCIE-SAN
