[HH] Butter
I have a gigabit network card in my NAS, I used the Netgear GA311.
For general transfers it's been fine

the only problem I've had is trying to stream High Def movies

and we (that's the royal "we" on the forum here) have come to the conclusion that the IDE controller on my motherboard can't cope with the speed of the network so I get a buffer underrun kind of effect

and slower transfer rates compared to the rest of the network.
In the next week or so I should be upgrading the m/board to one that is ATA 100 so hopefully that will ease the situation
So my advice for what it's worth is go ahead and put in the gigabit card, but beware that your m/board's IDE controller may struggle to keep up with the transfer rates. Considering the cost of a card about £15 in the UK putting in a gigabit card gives you bigger pipelines and can only benefit the whole network and help remove any bottlenecks transferring in and out of your NASbox.
Good luck