Building a Network storage device?

How much storage are you looking for? I personally like Snap appliances, but they are pretty expensive. I was going through the same dilemma, and decided to build a Shuttle box with Windows. For me, it's an all-purpose server, so it's a better solution than a NAS. You will find that the consumer-based products are all pretty slow, and all have their little quirks. I have never tried it, but there is a little Linksys device that has 2 USB ports that you could stick whatever size drives on. Beware of anything that requires a "client" to be installed on the PC. That defeats the whole purpose, in my opinion.

I'm looking for it to have a capacity of 250 - 300GB

You can buy usb, or rj-45 hard drives. it would almost be easier, though, to build another machine and setup a workgroup between all your pc's. doing that would be your best route. Also, linux boxes(preferable suse linux) run excellent samba servers, and are very reliable. For this, i recommend SLES9 (Suse linux enterprise server version 9). That would also cut down the amount of software you need buy by hundreds. But its alot more complex, and fun if you know what you are doing.