Dual channel output from your DIMM's requires full bandwidth from the DIMM slots. On a 4xDIMM slot motherboard DIMM1/2 will most likely be on the same bandwidth and DIMM 3/4 will be on a different bandwidth (I don't know the exact terminology).
To enable Dual on my Asus K7N8X I hae 3 DIMM slots and Dual Channel mode will not work if I have memory modules in DIMM1 and DIMM2 - most motherboards colour the DIMM's different to show this.
[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]