I am not sure whether or not your memory would still run at a CAS latency of 2.5. I believe you could sustain Dual Channel mode if you had your 2x256 sticks in DIMM1 and DIMM3, not sure what the affect would be to add a memory module to DIMM2.
I might try it though, I have 2x512 Corsair in DIMM1 and DIMM3 to get Dual Channel mode on my Asus A7N8X - Deluxe.
I could try taking my slow *** 512MB DDR333 memory module from my second PC and putting it in my system with Corsair, see what happens.
[size=0.5]Full Aluminium ThermalTake Case
AMD Athlon XP 3200+ *400 FSB* Barton (2.2GHz)
ThermalTake Volcano 11+ Xaser Edition Air Cooling
1024MB Corsair TwinX XMS 3200 (Dual Channel 2x512) 2-3-2-6
256Mb XFX 7800 GS EXTREME
2x WD Raptors (10krpm 8MB Cache) 36.7GB using RAID -0
Video Logic SonicXplosion 5.1 Soundcard[/size]