You could install Windows Home Server ontop of a RAID system but the Drive Extender and File Duplication technologies built into Windows Home Server is best taken advantage of with non-RAID setups. In RAID you traditional design your storage system, implement it and then stick with until disposal. Most RAID expansion techniques involve migrating data and in my opinion RAID can be quite cumbersome for the average home user.
With Windows Home Server you get the joys of dealing with a hetrogenous drive pool - that is drives can be mixed between models, sizes and storage types, i.e. you can mix up as many USB, PATA, SATA, eSATA, SAS, FC drives as your system can physically and logically cope with though there are some higher limits on overall pool size.
With Windows Home Server you can increase your storage as you need - with the right hardware you can do this on the fly. Repairing the drive pool is just as straight forward.
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