Dual channel with separate sticks

You can buy the second stick later and then run in dual channel mode. Just make sure to match the first stick exactly.

Dual Channel means you need 2 sticks otherwise you will cause problems for the bios, and the OS. Don't short-change your system, always check the motherboard specs for compatibility of ram.

Just get 2 x 4gb of ram, it's all you will ever use and need, ever. Usually, 1066mhz is the mid-range for ram but make sure you check your manual or the motherboard sepcs on the manufacturer site.

"you need 2 sticks otherwise you will cause problems for the bios, and the OS"

This is not true. Do not spread falsehoods around. Running in single mode won't hurt a thing except your performance numbers.