NAS Build Questions

[PhantmShado]

In looking at prebuilt NAS solutions, I've realized that to get any four drive raid box that can do transfer rates even as high as 10miB/s the asking price is about $500 before drives. Meanwhile I'm pretty sure that back in school we were reaching speeds of 100miB/s grabbing off a friends local 6 drive raid 5, so it seems some speed is evaporating here even for a 4 drive solution as it seems that a $500 solution should be able to cap out hard drive speeds (or at least get a lot closer). To this end I have some questions about building my own NAS:

1. Are my assumptions right? Can a home built box for this price do, say, 50 miB/s with a 4 drive raid 5?

2. A google search only seemed to come up with FreeNAS as an os for this purpose, would this be able to meet the stipulations of question 1?

3. In looking up information for this plan, I noticed that there is something known as RAID - Z (that free nas can run) that is supposed to perform better than RAID 5 for cases where you don't want to buy a dedicated RAID card (probably necessary to keep box size and cost where I want), is my understanding correct that this system can still tolerate and rebuild a failed drive?
3a. Would it be able to perform per my question 1 without a dedicated RAID card?
3b. Would it be recoverable in the event of a cpu, mobo, or FreeNAS os drive failure?

4. Is there a specific mobo/NIC I need to ensure my wanted speeds? The FreeNAS forums seemed to indicate that Atom D525 builds are the way to go, but they also seemed to largely be doing multi NIC builds. Is there an advantage to this I'm not seeing? I'm not sure why a NAS would need more than one NIC.

Thanks for any help!