I am considering getting a TYAN S2469 server motherboard.  However, I'm confused as to the power supply needed for powering this board.

Some webpages state I need a special power supply while on other webpages, it seems an ordinary ATX power supply will be fine.

" • ATX12V/EPS12V** universal power connectors"
VS
" • ATX & ATX 12V power connector configuration"


I have an Antec Neo 550W modular PSU.  Will this work with the Tyan Thunder?

To answer the Question, if it is this power supply : http://www.antec.com/us/productDetails.php?ProdID=28550

then the answer is YES

EPS-12V indicates the normal 24pin Main Power connector, the ATX12V 4pin Aux Power Connector and the EPS 8pin Power Connector (for Server Boards).

Notes from the Manufacturer (the Board apparently operates satisfactorily on a normal ATX 24pin + 4pin 12V PSU) :
http://www.tyan.com/archive/products/html/thunderk7xpro_spec.html
** Note: For heavy loading such as AGP Pro or multiple drive RAID systems EPS12V is recommended

Basically the Server-Boards can eat up so much power, that a typical 24pin+4pin PSU may not deliver sufficient power over the two Connectors, that's where the EPS 8pin comes into play.
With today's Power Supplies (quality ones), however, this shouldn't be such a big factor anymore on those older Boards.

Total power consumption should be in the Region of approx. 150-180W with an old PCI VGA installed and full CPU load. Add a better 3D VGA and the consumption will increase accordingly.

With current Dual Core CPU's, operating those old AthlonMP's is highy inefficient, a single cheap Athlon64 X2 4200+ EE will easily beat any Dual AthlonMP setup any day, at a fraction of the price and a far lower power consumption.

PS.
Fine Board being flexible like that, my Dual SocketF Tyan Thunder H2000m mandatory requires the EPS 8pin to be installed to prevent System damage.

-- edit --
http://www.tyan.com/archive/products/html/thunderk7xpro.html

Click on the red to view the example when powering it with a normal PSU.

-- edit 2 --
Be aware, though, that the AthlonMP FSB will always limit the Dual CPU setup quite a bit.
I've operated 2 Dual AthlonMP Systems in the past and their scalability was much limited by the RAM bandwidth. Mine also were Boards that could run off normal DDR RAM and did not require Registered DDR (Reg ECC RAM runs even a bit slower due to overhead when ECC is used)