RAID 1 doesn't provide a backup, it gives you drive redudancy, specifically in this case (RAID 1), drive mirroring. What this means is that all the data is written twice, to separates drives. In the case of a physical drive failure, the system will keep working in a "degraded RAID" state. This will allow you to replace the bad hard drive and rebuild the raid without losing your data or having much (or any, depending on the raid controller) downtime.
It's not a backup though in the sense that if someone accidently deletes a bunch of data from your MYSQL database, you can't go back and get that from your raid... the deletion would happen on both disks simulatenously. So you'd still want a backup. The backup is to protect your data integrity, the raid is to protect from physical failure.
For the NICs, you'd probably be fine with just the standard one that is builtin, but if you wanted to upgrade get the dell recommeded dual port NIC. That will give you redundancy in your NIC card as well.
Partitioning into two OSes will undoubtably consume more resources. What you are doing with the other portion of the server, and how much strain you already have will depend how much it effects your web server.
