Most of the consumer NAS devices have horrible performance. I would reccomend eSATA, an internal RAID, or RAID on another computer connected with gigabit ethenet.
I built a RAID array on another computer in my network with a 3 750GB drives and the performance over it with gigabit ethernet is great. Performance is practically the same as a directly connect drive. A transfer between my two internal 150GB Raptor drives actually took longer than a transfer to the network storage. I am getting 90-110 Megabytes per second with this setup
5DMK2 | 24-70 2.8L | 50 1.4 | 100 Macro | 70-300 IS (for sale) | 580ex II | 430EX