You won’t have the option of choosing between DDR or DDR2, the modules are not pin compatible and your motherboard will only support one.
In almost every usage scenario today, AMD CPUs perform better than similarly priced Intel ones. One of the few exceptions to this rule is media encoding.
If you are mainly going to be using the system for video editing get a motherboard based on an Intel 945 or higher chipset and the fastest Pentium D or Intel Core Duo CPU you can afford, otherwise go for an NVIDIA nForce4 for AMD motherboard and an AMD X2 series CPU.
For video editing the main performance factor within the system will be the amount of RAM (DDR/2) you have. Get at least 2GB, and then once you have bought everything else for the system, spend any change you have left over on more RAM.
You’ll find that most motherboards and hard disks will support most of the features of the SATA II specification (commonly 3Gb/s transfers and NCQ) as standard, so it won’t be a matter of ‘paying extra’ for these features. Having the extra bandwidth of SATA 3Gb/s will definitely boost performance when you are saving and loading large video files. The current sweet spot for large capacity/performance/price is IMO the Seagate Barracuda 400GB.
Most of the features of current graphics cards are focused towards 3D rendering and, lately, video decoding (but not encoding) so I don’t really think that it matters which you buy for your video editing system. I suggest you visit the forum hosted by the software company that writes your chosen video editing program and see if there are any specific recommendations there.
Ditto for the sound card. You may actually find that the onboard sound and graphics capabilities of the motherboard are enough for what you need and you don’t need to buy separate cards.
I don’t think it will be possible for you to find a motherboard that does not have LAN onboard. You will not need a separate LAN card.