Comment Thecus N5200 Pro & N7700 (Score 1) 517
To answer the poster's question, I have a Thecus N5200 Pro that performs pretty well - RAID0 should break 40MB/s - not amazing, but better than any other SOHO NAS's around, barring the new N7700, which looks like it'll hit Gig-E limits (finally a cheapish NAS that approaches 100MB/s for reads/writes). That said, you're gonna pay for the performance - the N5200 is about $700 and the N7700 is $1100 (enclosure's only). eAegis sells them w/ burned in drives as well - that's where I got my N5200. The hot-swap and automatic RAID rebuilding works as well and it has built in FTP, SMB, AFP, and NFS and is pretty good for a plug and play system.
That being said, you're definitely paying a premium, and you could easily throw together a multi-terabyte system that would max out your GigE for about the same price as what you'd pay for the N5200 enclosure. My only big recommendation there is that you get a hotswap rack w/ that - makes things much more pleasant when replacing drives.
One other thing to consider is power consumption. My N5200 Pro idles at 80W - if you built a low-power mini-ITX system you could probably get something pretty close to that, but a regular PC would probably be closer to 150-200W. Depending on your electric billing, you could be talking about a $100-200/yr difference there.