Hi,
I've been using FreeNAS at work for a while now and it does pretty well and its nice that you can install it to a USB memory key so that all 4 ATA connections can be used for disks (newer MB's only through for booting to USB key).
The main outstanding problem for me though is the bugs in the software raid i.e. if you have e.g. 4 drives at 80Gb each, it should be possible to present them as 1x 320Gb set (striped) or as 1x 240Gb raid 5 set (some redundancy). This config doesn't tend to survive a reboot on the hardware I've tried. Using them as individual drives works well, though.
Alternatively have a look at
http://www.openfiler.com/ which promises to do the same kind of thing. I haven't tried it yet but it looks pretty interesting. It also says it can do replication so if you are worried about losing stuff, you could have two
Both these systems are freeware and based of different flavours of Linux but the Network card support is based on what the OS understands and is being updated all the time i.e. Gigabit should be pretty easy unless you have really obscure NICs.
*** Update on OpenFiler ***
I've had a look at this and it is a bit too serious for home use i.e. it required an external authentication server such as NT4 domain controller, Active Directory, LDAP, NIS etc. unless you're also a networking enthusiast as well as MCE then its totally OTT.
Someone mentioned NASLite previously, how's that working out for you?
Cheers.