Matched DRAM -- Why? Is it important?

Crucial's website tells me:

Although the memory can be installed one module at a time, the best performance comes from using matched pairs of modules.

What exactly does that imply (what exactly needs to be "matched"? the brand, size, speed, etc.?) and how important is it for Windows?

Oh, and forgot to add: Does that imply it's better (in terms of speed) to have a single 4GB PC3-10600 module, rather than a 4GB PC3-10600 + 2GB PC3-8500 combination on my laptop?

Okay, I just bought a new DDR3 module, and now my configuration is:

• One PC3-10600 DDR3 module (4 GB)
• One PC3-8500 DDR3 module (2 GB)

and the system seems to be fine so far, and my Windows Experience Index score went from 5.9 to 6.8.

So I guess there isn't much of a problem with mix'n'match'ing! :)