I would like to purchase a RAID controller that is Win7 compatible, has external SAS or eSATA connections to support 8 drives in an external enclosure, and allows for online dynamic expansion. I was looking at the new SATA-III cards from Adaptec, and they look great, except the ones I saw lack external support.
Try LSI.com. you will find any type of SAS/SATA controller available. Note that any SAS External Port will be x4 Lanes and will support 4 drives directly or goes to an expander on the drive enclosure to allows for many, many drives as well as cascading drive boxes.
With the prices of the controller cards being either very expensive or pretty reasonable it is going to depend on what your budget amount is. The more money you have to spend the more features that are included. For what you want this card would work;