Take a look at the two pictures...http://www.msicomputer.com/product/server/MS-9620.jpg
Thunder K8WE (S2895)
You'll notice that the Tyan board has three chipsets. One is the PCI-x tunnel, which doesn't really matter that much.
The other two are the 2200 and 2050 nForce MCP chips. These allow both PCI-E x16 slots to actually run at x16. Also, they are professional chipset products and are designed for stability.
You'll also see that the Tyan has two banks of DIMMs. This means that each card has it's own memory bank, and can therefore use NUMA. (non-uniform memory architecture) This will mean faster memory performance in applications as the processors will store data in the memory banks they can access the fastest - the ones they are directly attached to - instead of loading it anywhere and having to go through the other processor to access it's memory.
On the other hand, the Tyan does not have PCI-e expansion slots beyond the two x16 slots. So, next gen PCI-e soundcards, etc... won't have a place to go.
The MSI has two "x16" slots. In other words, either one is operating at x16 or both are operating at x8. Also, it seems to have only one bank of memory, so NUMA is out, and the second processor (or third and fourth if you go dual core) will have slower memory access since it has to request access through the first processor. It also uses the professional chipset, but doesn't buddy them up. It just uses the 2200 hence the 16/8 switch. But, it does have a single lane PCI-e expansion slot.
They are both great boards. The deciding factor is how you'll use them. The TYAN would be my choice because I do workstation stuff - rendering and animations. So, graphics performance and stability are crucial. I don't even own a sound card, and the last expansion card I bought was an ethernet card for my micron pentium 3 piece of junk.
If you're going to be using the system for personal use, gaming, fun, seti@home, etc... then you are going to be quite happy with the msi most likely. The memory performance will be fine, the processor performance will be stellar (though not quite FX levels), and the graphics performance will be equivalent to anything available in the Athlon64/FX lines. If you're into professional work or meddle with really big files and multitask like a crazy person, then the Tyan is worth the money. However, I'd say that maybe 1% of non-professional consumers would actually strain either board enough in their daily usage to notice the difference.
So, there you have it. Never a straight answer.
Iwill Zmax DP SFF Barebones kit... (300W PSU, 8 USB, 1 FW, etc...)
Dual AMD Opteron 275's | 2 Gigs (2 Dimms 1024Mb) Patriot RAM 3-3-3-8
WD Caviar 80 Gig HDD - System
WD Raptor 74 Gig HDD
ATI Radeon 9800 Pro 256
NEC ND-3520A Dual Layer DVD±RW | Mitsumi Floppy/Card Reader combo