About mainboards, MSI is based on VIA chipset while Tyan is based on AMD-8000 chipset. There is not much difference comparing the two chipsets: Check the benchmarks below comparing the two mainboards:
http://www.3dchips.net/content/review.php?id=67&page=7
http://www.amdmb.com/article-display.php?A...48&PageID=2
http://www.3dchips.net/content/review.php?id=67&page=23
Either one would be a good choice, althou Tyan gives you more memory slots for future memory upgrades.
About the video cards, I would advise strongly for Quadro FX 1100, it's believed to be the best bang for the buck and if you check the benchmark below, you'll see that it even ties in some Maya tests with FX 2000 and 3000, even if you rub Fortknox, I would suggest FX 1100, because honestly, there's no point to paying more and getting the same performance.
Video card benchmarks:
http://www.3dchips.net/content/review.php?id=63&page=17
To find an Extended ATX case for Tyan, check the link below, Antec has a good case which is not too expensive:
http://www.epinions.com/System\_Cabinets-Extended\_ATX
or just search http://www.pricewatch.com for Extended ATX cases.
And since you're on a mission to build a workstation and post as many questions as possible, I'll just answer your next question 
About hard drive, you can either go SCSI or SATA. SCSI is more expensive and more reliable, SATA is cheaper and gives you the RAID option. There are also 10k rpm SATA drives. You can get a 36GB SCSI 10k, OR, get two 80GB SATA and put them in RAID0, the price would be somehow the same, RAID0 thou is a bit risky, becasue if one drive fails, you'll lose all data, but personaly, I haven't seen a SATA drive gone bad, as long as you stick with good brands, like, Western Digital or SeaGate. Check their website to see their pricing.
The best combo for your drive thou, would be using one SCSI 10k or 15k drive for your application and another SATA 80GB or higher for storage.
About monitor, ViewSonic is believed to be the best, but it's also expensive, my personal advice is that, go for something that supports higher resolutions with higher refresh rates (70Hz or higher), if you plan to spend a lota time working, it wouldnt cause too much headaches.
Also, if you decided to go Opteron, make sure you install at least two 90mm fans, three is even better, Opteron chips would get really hot during Rendering and with a poorly cooled case, you may lose one or both of them.
Make sure you get a DVD optical drive as well, since most Maya learning stuff are in DVD format these days.
About Memory, Corsair and Kingston are good brands, and since you need to install them in pairs, I suggest you go with two 1GB. Of course, 4 x 512MB would be cheaper but also will not leave any room for future upgrades. If you're on Windows, more than 2GB of memory is practicaly a waste of money since Windows wouldn't allow more than 1.5GB for Maya or other applications to use, if you're on Linux, you can install as much memory as your mainboard supports, but, nobody knows how much memory Maya is able to use!
The power supply is also critical, I wouldn't suggest anything less than 400W, thou a 550W is better. Most workstations manufacturers like IBM or 3DBoxx, have a 530W (or higher) power supply on their systems.
I can't think of anything else, so there you go and good luck!
Michael
