Given two memory slots, what is the optimal configuration given my 2x1GB, plus either 1x4GB or 2x2GB? (I just bought a used laptop with two slots, currently 2x1GB, max 8GB)

Possible upgrade combinations (assuming permutations aren't necessary -- does slot order matter?):  

3GB = 1x2GB + 1x1GB  

4GB = 1x4GB  

4GB = 2x2GB  

5GB = 1x4GB + 1x1GB  

Which is my optimal configuration?

I am looking for clarification in these concepts (explanation why is appreciated):  

a) Is 2x2GB better than 1x4GB?  

b) Must the two slots have exactly the same capacity to gain the benefit?  

c) How does 2x2GB = 4GB match up against 1x4GB + 1x1GB = 5GB?  

d) Does 8GB max mean 4GB max per slot?  

e) In theory, given the max 8GB, which of 1x8GB and 2x8GB would yield better performance? 

Edit:  

I looked into dual-channel memory and from my understanding it reduces the latency of some operations and allows for concurrency during others, resulting in a marked performance increase that is not possible with only one chip. (Thanks David)