Dual Channel Memory Question

I took the plunge and bought the following;

Aopen AK79D-400VN NForce2 Ultra 400 MB
AMD Barton 2500+
2x256MB DDR400 PC3200

Everything works excellent. Right now, as I'm typing this, it just got done
it's burn in (Sandra) at 11x200 (2.2 GHz or 3200+ speeds) at a maximum temp
of 47ºC. Average was 44.5ºC. This is with the original AMD HSF! Can't wait
for water cooling! =D

My only question is that I bought 2 sticks on RAM for dual channel support
but when I populate DIMM1 and DIMM3 it won't even POST. It just sits there
beeping one long beep which I assume means there is a memory problem. Is it
that dual channel needs all the slots populated? If so I will go out and get
a 512 stick to populate DIMM3. The only way I can get it to POST is with the
256 sticks in D1 & D2. I am not new to computers, just the NForce2 chipset.

Thanks for your help.

Just in case my systems is:
AMD Barton 2500+ at 3200+ (11x200)
2x256MB PC3200 DDR400 in Dimm1 and Dimm2
Aopen AK79D-400VN
MSI GeForce4 Ti4200 128MB (300 core/530 memory)
Creative Soundblaster Live! 5.1 (Need the line in and mic connectors w/5.1
speakers and onboard sound can't do it)
WD 80GB
Seagate 15GB
LG 40x12x40 CD-RW
8xDVD-ROM