If I recall the 221 is a single channel controller. That being the case you can use it for the internal drives or the external enclosure but not both. To use on internal drives you would have to move the internal connector to the 221 array card, but that would prohibit use of the external drive array.
You might consider going to ebay and getting yourself a 3200 card. You can pick them up for 5 to 10 bucks. Be careful in what you get, the 3200 come in 16 to 64 meg of onboard ram. If you use a 3200 and if it has a daughter card, you have to remove the daughter card (card near the connectors) if you plan on using one channel for internal drives and one channel for your drive enclosure.
Your question 2 on the OS is a little more involved. First you should get the drivers setup in the os before changing anything. Simply install the board in the server as is bot the unit as normal and load the drivers in the os before you change anything. Once drivers are installed disconnect the cable from the existing controller and attach it to channel 1 of the new array. If the drives are already setup in an array, the 3200 will pick up the settings from the drives.
Use smartstart version 5.5 and setup the other channel on your enclosure.
Good luck,
Phil