I run an Ubuntu server on a fake-RAID5 and it works great as NAS.
The RAID5 was configured through Intel Matrix Storage Manager (chipset dependent), but to mount it in Ubuntu, I used mdadm (lvm is a very powerful alternative, dmraid another simpler alternative).
This is problematic when initially trying to install the OS, as mdadm/lvm/mdraid don't seem to ship with Ubuntu's standard OS download (only gparted). However, I believe the "alternate installs" work fine at installing onto a RAID. I ended up doing the extra legwork and installing the OS onto the RAID5 partition (dual boot, other partition containing Windows) using the debootstrap / chroot method from a livecd, which is a bit long-winded but I think worth the experience of the exercise. If your NAS box doesn't have a CD drive, then one option is to mount the RAID onto a computer which already has linux up&running, and chroot from there.
In terms of Network visibility, as mentioned above NFS is fine from Unix, and can be configured for Windows clients as well (home editions might not work, and enabling it is one of those things that only techies know how). So for the best windows compatibility I'd set up a samba server, which also works fine from Unix machines too.
Webdav is pretty cool (enables writing over http(s), setup through Apache, and mountable remotely, including through a proxy / firewall), but doesn't allow streaming commands (e.g. play,pause etc.) for video & audio files. (It's great for working with shared documents though.) Instead an RTP-compliant server is best for network-attached media. VLC has an RTP compliant web interface, but I haven't seen that work yet. Other commercial options are available to try / buy.
So I'd recommend installing Ubuntu Server on to the array and using mdadm to manage the RAID. It works great for me, although I haven't needed to do anything technical with the array (e.g. rebuild a drive). If you need remote access and it's behind a firewall, then I'd setup an Apache webdav location, which is totally visible via a proxy. If it's on a corporate network, and you want everyone to have access, you might want to try LDAP authentication. Otherwise Digest authentication is good, although you'll still want to try & enforce https over http, which is done in an Apache VirtualServer directive:-
RewriteRule ^/webdav/?(.*)$ https://%{SERVER_NAME}/webdav/$1 [R=301,NC]
That's my 2cents... Have I gone off topic??