Building your own case? Yikes. That would be a challenge. Well, it obviously depends on how much space you're after. If you've only got 3 drives, that's pretty easy and your cheapest solution would probably just be a regular case. Server, rack-mountable cases tend to be more expensive. I bought mine on Circotech (google it). They had very good service. Unfortunately, a small cable was missing when I received it but they shipped one out promptly--granted after they were gonna ship it I added two items to my order and total was ~$100. He claimed he was giving me free shipping but I'm not sure because the two items were not listed on their site (more special, less common items). I was getting the bezel and rails.
Technically it's 7TB but the difference of 1024 vs 1000 and other differences gives me just over 6TB. That's what makes me want to add a few more drives to get 7TB but then I'd technically have 8TB, lol.
Tell me what you want stored, how much data expansion you want, how much backup you want (remember DVD backups + hard drives work very well), your budget, etc. Do you want some higher end features including hot-swappable fans? My case has those...not that I'll ever need them! It also has hot-swappable SATA bays. You probably don't want a redundant power supply. Lastly, what other uses will this server have? What OS? Are you going to run more applications? I personally run a web server, email server, sometimes FTP server, Media Center's UPnP & Library server, file server, security camera recording for FOUR cameras (D-Link), Juice podcast downloader, Syncback to auto backup from 7TB array to smaller array in the same case, etc.
So I have quite a few demands other than just files. My web and email servers aren't heavily used but are definitely used consistently throughout the days. In fact, I listed 15 items or so on eBay and all the pictures are hosted on my server. I tend to post lots of very large pictures (800px wide, etc). One auction has almost 2MB of images, which is a lot compared to what most people have. I never buy the extra images from eBay...I just host them for free on my server which only costs me $30 per year for the dynamic IP redirection & domain name (I have a few more domains, though).
Wow, I'm off topic. Just tell me more about your intended server. It's a lot of fun building a server because you know that your efforts can benefit you for a few months (lol)...maybe YEARS. My server has kept all my data online as requested (restarts and down-time mainly by choice) for almost FOUR months. Yes, that's not much time but that's since I built it!