The most important question in this is what existing I/O do your 200 nodes already have?
The reason I say this is that many compute intensive rigs use infiniband and if you've already got that infrastructure in place I'd strongly advise you to simply buy an FC-to-IB gateway and 'sweat' your existing investment - it'll be fast and resilient.
In the absence of IB the next best performant SAN would be an 8/4Gbps FC network - this will be super fast but very costly for 200 nodes as FC HBAs/SFP+s/FC-switches etc aren't cheap at that volume. That said this is a great solution that would let you sleep well at night. Oh and I agree with Adrian above that HP EVAs are super easy to setup.
If your nodes only have 1 or 10GB ethernet then I'd be tempted to go for either one of these NAS-like distributed filesystems such as gluster or just a plain old mid/high-end NAS such as a NetApp (smaller ones will really struggle with the load). This'll work but you may see issues at peak load times, of course it means your costs will be far lower than the other solutions. If you go the 'big NAS' route I'd be tempted to hook them up with 10Gbps NICs as their price have come down a lot and they'll be much easier to deal with than lots of teamed 1Gbps NICs.
What I would absolutely avoid is iSCSI, the reason is that if you wish to have any shared storage, even just two nodes seeing the same volume, you'll need a cluster file system - these are fine for a few nodes but won't work well for 200 nodes - if at all - the arbitration alone would cripple the environment. This is less of an issue with FC by the way as the arbitration is designed into FC more.
Hope this helps, feel free to come back with follow up questions. Oh and good question by the way :)