Dual Channel Advise/Help

Hi all,
can you somebody advise on how dual channel memory works? I have a Gigabyte GA-8N-SLI Pro motherboard which supports dual channel memory. Initially I had 1 stick of PC5300 1GB memory. Recently I purchased another 2 of these memory sticks on the cheap, so 3 in total.

My questions are

1) To avail of dual channel memory, should I only install 2 memory sticks, 1 in each of the 2 same coloured memory slots or can I install all 3
2) If I install all 3, will this prevent 2 modules working in dual channel?
3) Do I need to enable dual channel in the BIOS or OS for it to work?
4) I also have a graphics card with 1GB of memory. I now notice that with everything installed that "Physical Address Extension" is mentioned in the Systems->General tab. Does this slow the PC down? Should I remove 1 memory module?

Cheers
Shav

Thanks O1die.

Right now I'm in process of slowing upgrading my PC. I have a 8800GT card but only a P4 630 3.0Ghz CPU. Therefore, the large bottleneck in my system when it comes to new games is defo the CPU. I was hoping that the dual channel could improve my FPS.

Is 3GB RAM really a requirement for XP. If I got 5-7% improvement on FPS for games based on 2GB dual RAM as opposed to 3GB single RAM, I'd be happy with that

Cheers
Shane

Cheers guys