Hi, I have just taken the plunge on what I thought was a super-computer for Autodesk Inventor.
Xeon 5645 CPU, Intel Workstation board Shady Cove S5520SC as you are considering.
Firstly, comparing the two mobo's that you are considering...
I thought "Server" technology would be bullet proof - compared to desktop technology - (Core i7 processors - which are a derivative of the server developments) . On "paper" the S5520SC looks to be a far superior choice with huge expansion potential.
Looking at compatability of your 5645 cpu with the DX58SO2, the link below suggests that the mobo is not compatible.
Practical aspects are summarised as follows:
1. You can start with 1off Xeon cpu as proposed. It allows 6 ram slots - the other 6 are for the 2nd cpu and are thus out of play for now. So you need to go for big ram 8GB modules or more.
2. You need to be sure about the chassis that you get. It must be compatible with the mobo. (Do NOT go for Hanlan Creek mobo etc. they must be server technology Workstation mobo's - as you are considering). The chassis will end up being one from hell - unless you are in to heavy duty IT and are familiar with chassis. It will cost a bit! But it is necessary. They are professionally constructed etc. - and you will need a strong back to move it around.
3. You need to get a "proper" power supply. Spend the few extra dollars to go for a good one - you need lots of ports / cables to power everything - e.g. if you install a 2nd cpu, the power supply needs to be able to offer a 2nd power point to the mobo etc.
4. Cooling. If you go with water cooling kits you may run in to space problems ( if you use RAM with very large cooling fins you will certainly run in to space problems.) You could use standard air cooled heatsinks if your office has airconditioning.
5. I think you are correct to go with ECC ram. The above will work with all of that - i7 type mobos won't - (in general)
I added SSD, 8GB ram modules and as of today I feel conned.
I had to revert to my "old" machine to load Win 7 x64 SP1 - the Xeon got to a point and hung when installing - like for hours. I tried installing on another new drive - V.rapt - used them for years on my "old" machine - same problem - hangs when installing windows on the Xeon.
The CD with the mobo above got to a point and said that it could not read the OEM files from the disk. Downloaded something from Intel to install mobo drivers. Don't know what it was because the machine is effectively useless. Apps. take ages to open, if they ever do. Some won't open at all. Can't run the Win 7 performance diagnostics to see how "fast" it is because the app. just hangs. (No programs are installed yet!)
I wish you better luck!