They offer a NAS addon. Its not Raid 5 but their own disk mirroring solution. The base version can give you up to 3TB of usable storage if you use 4x1TB disks, and is easy to upgrade plus you dont have to match drives. They also now have a higher end system available its a bit more costly, but takes 8 drives instead of four and allows you do configure a 2 drive failure redundancy (a kind of RAID6) if you wish it.
Obviously it depends on how many drive slots you need but for personal use I'm a big fan of Thecus's products - I have a 5200pro and as it's *nux-based you can extend its functionality quite heavily, certainly with the protocols you're looking for.
Last time I was in a shared Win-Mac environment, this little Terastation took care of joint file space. Never had the slightest issue with it. The thing kept itself happily cooled, made no noticeable noise, and had more options than I could shake a stick at.
I have requirements similar to yours and when I went looking all the solutions I found were either underperforming (ie. too little RAM/cpu to be able to do anything other than serve files) or overpriced. I ended up building a mini-itx system into a bookshelf case with hotswap bays. I chose one of the new dualcore atoms for lowish power and boot from an sdcard in a usb adapter so that the disks are just for storage. I've got plenty of spare cpu and RAM to be able to run mt-daapd to serve out the music subdir as well as other 'internal' apps.
Friend of mine is using 2-bay D-Link DNS-323, and he's quite happy with it. There is larger 4-bay version (DNS-343) that would fit your needs. It's OS is μClinux based, so should have everything you need.