The actual total capacity you need depends on the applications and operating system you are going to use. They normally does not affect the speed of the system apart from the simply fact that more memory means less paging and faster response. For a typical computer today running a 64-bit OS, you need 4GB of RAM.
The configuration of the sticks slightly (marginally?) affects the speed of the memory sub-system. The exact requirements are mother-board specific, but the general rule is you need an even number of sticks that are exactly the same to use dual-channel.
Some motherboard/BIOS allows you to use different ones, but many laptop don't. So you need RAMs that have the same capacity, speed and bank configuration (i.e. how the chips are grouped). Some motherboard can enable dual-channel with a single dual-bank RAM, but I won't count on that.