NAS boxes are way overpriced
IMHO for what they are - low powered CPU in a plastic box with some empty HDD bays and a proprietary or Open Source operating system
You could just buy something like
http://www.amazon.co.uk/ProLiant-N40...9070490&sr=1-1 add the hard disks you mention and run something like:
1) FreeNAS 8
2) NAS4free
3) Open filer
or if you want to pay - Windows Home Server
Net work performance - just ensure you have a gigabit ethernet card in the machine ( and use gigabit switch and CAT 6 cable)
All above OS support Raid