First off I would like to say thank you in advance for any/all suggestions.

What I am looking to do is setup a an external drive on my network, to allow my three computers, my wives iPhone 4s, my 1st gen iPad, and PS3 to access any files I place on it. I would like to centralize my iTunes library and iPhoto library on this drive, so that all the computers, the iPad, and iPhone can all use those libraries for viewing, editing, and adding. I would also like to setup folders for myself and my wife, and eventually my kids, to securely store files on the drive. I would also like for the external drive to be a dual drive setup, with RAID hardware.

The setup I have is:

Apple Airport Extreme 802.11n (1st Gen)
21" Apple iMac (Running Leopard)
13" Apple MacBook (Running Leopard)
13" Apple MacBook Air (Running Lion)
1st Gen iPad
iPhone 4s

My questions are: Should I just get a dual drive enclosure and connect it to the USB port on the AE, or should I go with a NAS system of some time? If I go with the NAS, can it be plugged directly into an ethernet port on the AE or do I need a switch? Please feel free to recommend any specific hardware, setup steps, and/or advice from experience. I currently have all the music and photos on my iMac, and back the whole system up to an external drive. Once I have this shared drive setup, I plan on getting rid of my iMac and getting another MBA to hopefully alleviate the need to have a desk taking up space in my home. Also, has there been much improvements made to the hardware of the AE since the generation I have? If so, should I also look into replace the AE with a new one, or wait and buy an 802.11ac router? Also, would it be easier to setup this shared drive using an AE or with a Cisco router?