Using a Raspberry Pi seems to make alot of sense if you're in the market for a small NAS server, it's low cost, low profile, low energy etc...
Is It Up To It?
The only possible problem is the storage the only way to attach any real storage to a Pi is via USB. USB 2.0 has a maximum theoretical throughput of 480Mbits/s with a but I dont think you'll ever get anywhere near that speed.
We only really need to acheieve around 10MB/s as then the ethernet connection becomes the bottleneck
I wanted to see what kind of throughput I could actually get with USB devices connected to a Pi so I wrote a device benchamarker tool that is able to measure both the read and write speeds of one or more block devices.