Firstly, Hi Welcome to the forums.
This is a very interesting use case. Typically most people who come asking about servers are looking to run other games, not one they have made themselves.
Anyway... cases. Norco make some really nice cases. Although typically large (some mid) tower cases can fit Dual socket motherboards (SSI EEB Form factor). If you are looking for "rack-ability", then go Norco.
Motherboards... as the CPUs you have are LGA 1366 you are kinda limited to what is available. Try looking for Supermicro boards, I can personally recommend the X8DTI-F (have several) and the X8DT3-F (if you want SAS on the board). They are rock solid boards of the (approx.) 20 I have bought for various machines, only one had a fault and was quickly replaced.
If you are looking to fit it into a 4U chassis, you can use Cooler Master Hyper TX3 Evo. Very similar to the 212 evo, except they use 92mm fans instead of 120mm making them fit into the 4u form factor. If you are going to a tower case, then you can the larger 212.
HOWEVER, as most (if not all) the LGA 1366 server motherboard come with an integrated backplate (like LGA 2011) and require a bit of creative thinking to mount heatsinks not designed for the motherboard to it. (Or look for the super micro ones)
For the RAM, as it is LGA 1366 and uses triple channel, you'll get better performance by stepping upto 48GB of RAM (so that each channel has 3 DIMMs populated). You can easily get away with some cheap UDIMMs (non-ECC), although you may wish to step up to some Hynix or Nanace Registered ECC. If you intend to hammer this thing with data and continuos uptime.
GPU for rendering... well which technology do you use. OpenCL or CUDA? OpenCL == AMD, CUDA == Nvidia.
Power supply. Either look for a server grade one(with redundancy) or if using a standard ATX one, makes sure it has 2x 8pin EPS for the CPUs (as each CPU will require an 8pin if going under heavy load).
Other than that, personally I'd say look at a newer platform, like an E5 Xeon on 2011 or the i7s on 2011-3. Their Single threaded performance is far superior to that of the LGA1366 Xeons. You can still easily use the LGA1366 Xeons, I would focus on threading your game engine though to get the best performance.
Now for my nosey part... Typically server side on games doesn't really require much in terms of performance (except minecraft) as all they do is use the server to share data about other players with each client connected. Typically any physics calculations are done on the client side. You just have me curious in this decision in coding is all.