1. you need two NICs. this system will become the center of your house, replacing firewall, gateway, and dhcp to provide the services you're requesting.
2. your front-ends need to have nvidia based GPUs. ATI is somewhat broken at the moment. I recommend the MSI Media Live for an MD. works great out of the box. For video cards, an NVIDIA 6200 or better is strongly recommended.
3. don't do the dedicated raid 5, just connect all the disks individually, and configure the RAID through the web admin panel.