I use an Advent ReadyNAS NV+ It is not as fast as the Firewire options that are available (such as the Wiebetech RAID systems) but with Gigabit ethernet read/write time is reasonable, especially if back up is done automatically.
Just make sure you use a Gigabit switch and Cat 5e or Cat 6 cables.
Despite the slow speeds of NAS they do offer functionality that the Firewire systems don't. If you're on the road and only need to download a few images for your clients, then access speed shouldn't be an issue. Buffalo also offer good systems. One issue with NAS is noise, I believe that the new Buffalo systems are some of the quietest around.
If you have a cold room that needs heating, some sound insulation and deep pockets then there's always Apple's XServe RAID.
I looked at LaCie but was strongly warned away by my dealer.