I'm not sure gigabit would be worthwhile here. Wireless N is meant to be good for 300mbit iirc, but I don't think this is a figure you'll ever see. Particularly if the drive is a fair distance from the router with ceilings / walls and the like in the way. So I'm not sure it'll be possible to take data off the drive faster than 100mbit would allow.
The performance, and whether it's good enough or not, rather depends on what you're doing with it. It'll stream music all day without a problem, but if you want to stream 1080p to several computers at once you're out of luck. If it's backups you're good to go.
I've been quite happy with a nas over 100mbit, but I've not tried using it with more than two computers at once. I have a suspicion that it would thrash if I tried this.