The Asus website informs me that the board will accept a max of 8Gb with a 2Gb stick of either ECC (since it's a server board) or non-ECC (standard desktop RAM).
The Asus memory list (QVL) of course list's no 4 or 8Gb kits- not an unusual occurance with Asustek.
OCZ's memory configurator lists no 4Gb kits as compatible-only 2Gb kits. So the option here is buying 2 kits and populating all four DIMM's
http://www.ocztechnology.com/ > Tools > memory configuator
OCZ Platinum DDR2-800 2x1Gb part no. OCZ2P800R22GK
OCZ Titanium XTC DDR2-800 2x1Gb part no. OCZ2T800IO2GK
These kits have an embedded optimal profile (EPP) that the motherboard can read and set automatically:
OCZ Titanium XTC EPP Ready Edition 2x1Gb part no. OCZ2T8002GK
OCZ SLI Ready 2x1Gb part no. OCZ2N800SR2GK
All the OCZ kits have equally good timings (CL4)
Crucial memory have 4Gb (and 2Gb kits ) available (timings equal to OCZ)
http://www.crucial.com/store/listparts.aspx?model=M2N32 WS Professional
G.Skill
http://www.gskill.com/configurator2.php?pid=2&model=1047
Offer both 4 and 8Gb kits (also at good timings)
Corsair compatible RAM;
TWIN2X4096-6400C4DHX (2x2Gb DDR2-800, CL4, Dominator series)
TWIN2X4096-6400C5DHX (2x2Gb DDR2-800, CL5, Dominator series)
TWIN2X4096-6400C5 (2x2Gb DDR2-800, CL5, XMS series)
Q2X8G6400C4DHX (4x2Gb DDR2-800, CL4, Dominator series)-likely very expensive
Kingston have one RAM kit suitable for your motherboard
(2x2Gb) KVR800D2N6K2/4G (DDR2-800 CL6) a very very average kit
All these manufacturers offer a lifetime warranty.