I tried a zyxel NSA310 and had to send it back as it was sooooo slow. It took about 8 hours to copy 16gb of data to it. Is that normal?
No, that's not normal. By my calculations, that's approximately 0.57MBps (megabytes per second). If you didn't use wireless and were all wired, with a gigabit router let's say for argument's sake, you get 94MBps (752Mbps - megabits per second), that'll mean to transfer 16gb of data, it'll take you 2 minutes and 54 seconds. However, other factors can affect how fast you transfer data at, e.g. Hard Drive speed.
By getting an N WiFi router, if the computers which are connecting to it are nearby, I'd recommend running it on 5Ghz so long as the wireless adapters are also N compatible, otherwise the clients running on G or B will lower the maximum transfer rates for ALL clients, including the N clients. From an article I've read, 5Ghz (advertised at 300mbps, most likely to get 180-200mbps) was 20%~ faster than running on 2.4Ghz (around 150mbps), however this can vary I should imagine. If however the computers aren't close to the router, run on 2.4Ghz as this will provide a better range than 5Ghz.
So, with a NAS your options are as followed:
All clients wired, including the NAS to a gigabit router. Should be able to transfer 16gb in approximately 2 Minutes 54 seconds. Can vary dependent upon other factors.
Wireless N router;
2.4Ghz - Lower speeds, better range, more suitable if you're already having issues with signal strength strength. Transfer 16GB in around 14 minutes and 30 seconds.
5Ghz - High speeds, smaller range, better if you're not already having issues with signal strength. Transfer 16GB in approximately 11 minutes and 30 seconds.
Hopefully this will help you. It's been a long time since I worked on my CompTIA N+ (damn, I should've kept with it, I'd almost finished the video course).