Thing is though, that the internal HDD kits are cheaper than external enclosures of the sort needed to work with proper RAID cards. Of course this only covers 1 - 4 HDD's, but such accessories aren't exactly peanuts.
I would suggest looking at Sans Digital for an external enclosure (
example; notice the ports on the back, as they do NOT connect via SATA connectors, but a particular type of SAS connector).
The enclosure won't care about the OS, so that's not an issue. The RAID card selected however, would need to support both OSX as well as Windows. Also note, that you could find a card that may work under OSX, it won't be able to be a boot location (Areca's OSX compatible cards will boot with a firmware flash, which is yet another reason to recommend them).
Please understand, that RAID isn't actually all that simple to implement, and I HIGHLY suggest you take the time to research what it is, and pay particular attention to the different levels (start with Wiki).