While I agree with most commenters that you need to supply many more details before even beginning to narrow the options, if you do look at the storage vendors, DDN (Data Direct Networks) is really hard to beat.
I see the EMC Isilon guys posting here and need to counter.
I've organized bake-offs between DDN, Isilon, and a number of other vendors. DDN always came out ahead on price and performance (every time they were half the price and twice the speed as Isilon). DDN is the most represented of the vendors on the Top 500 Supercomputing list and also power a certain streaming movie/TV service we all know and love. DDN is also a pretty ethical - if they're a bad match for your application, they'll let you know and provide recommendations.
Whatever you do, don't build it yourself. As tempting and fun as it is, given that you're asking the question, you've already self-identified as someone who won't be able to support it. I've seen many smart people go the SuperMicro JBOD route only to create support nightmares for themselves.
Also, for that much space, avoid Amazon at all costs. It's way too expensive compared to dedicated hardware.
For cost, budget around $150-250k to get started. It might seem pricey, but you'll spend more than that on manpower building it yourself (or your first few months on Amazon).
In addition to DDN, IBM, Dell, and HP all have solutions in this range that aren't terribly expensive.