Still, why is SisSoftware Sandra only reporting 1 memory stick for his?
He mentioned in his post he's not using the dual channel setup anymore. That's probably the reason why it's reporting 1 stick.
As for Dual Channel DDR, nforce2 boards with 3 slots do in fact seem to run in Dual Channel mode with 3 sticks of ram.
Nforce2 boards seem to have a maximum support of 6 banks of memory to run in DC mode.
The reason why 2x256megs in one channel and 1x512 megs in another channel would be slower is usually due to the latency of higher density chips generally being higher than the lower density chips.
Since memory controllers generally can't run with split timings (least not to my knowledge) for each individual channel, they all generally have to run at the most common stable speeds.
Of course if your 1x512meg stick can run at 2-2-2-11 (the somewhat tested optimal timings for nforce boards), then odds are you'll be able to run all 3 sticks at optimal settings.
[edit] well manufacturers generally list something like this for nforce boards :
Supports 3DIMM's DDR 200/266/333 (Max 3Gb)
Supports 2DIMM's DDR 400 (Max 2Gb)
Implying that a 3rd dimm running in dual channel DDR would infact risk instability. If you ask around though I wouldn't be surprised if someone did get an uneven configuration working at DDR400.
The intel boards seem to be the ones hard wired with the matching pairs rule. Though I can't speak from experience since I haven't touched an intel desktop pc in ages.
My new Dell PC has "2GB Dual Channel DDR2 SDRAM at 533MHz (2x1GB)" - if I use it as dual channel, will my PC think I only have 1GB of memory, but that memory will be faster than normal?
No it should still recognize it as 2gb of ram.
