I have setup some clients for CAD in a scenario just like you mention.
These guys have very large files and while they used to copy the file locally to work on it and then copy it back BEFORE they had a gigabit network, once they upgraded they no longer copied it locally to work on it.
We set them up with a ReadyNAS and they were very happy.
Naturally, the more you spend, the happier you will be. Cannot say for sure that you will find performance good enough straight off the NAS, it's one of those "Your Mileage May Vary" things.
A ReadyNAS Pro Business will offer great performance, 6 total drive bays available.
I have a review here:
http://networkstoragetips.com/netgear-readynas-pro-busi...
The Pro has a high power core duo CPU and, since it's a business class unit, 5 year warranty.
Best,
Roger.