Future new owner

Hello all. I finally broke down and ordered two AW systems to replace my aging/dead computers at home. I got what I feel to be a really fair deal on a nicely spec'd M18x and M14x (for traveling). They are both in production now and am hoping that by the time I get them, I will have the mSata and SSD I ordered for the M14x. I do have a few questions about being a new AW owner. (First new PC in over 5 years).

For starters, I only have 1 year complete coverage warranty. Can that be upgraded at a later time for a fee? I'm sure it'll be a lot, but I had to get within a certain budget, so I lowered the warranty for the time being.
Second, I'm going to be installing my copy of Win7 on the two systems shortly after I get it. What is the correct order of installing the drivers and applications from Dell?
Third, before I begin any of my own modifications to the systems, I'd like to stress test them to make sure everything is good to go, that way once I start with my Win7 Install, I know that by the time I'm finished, everything's in working order.
Fourth, both machines are coming with 6GB of memory, which I feel should be enough. I'm not going to be running multiple instances of VMs or anything like that. But in the case that I want to upgrade the memory is there any downside to running the 6GB kit alont with a 4GB kit?

Sorry for all the noob-ish questions. Just want to make sure I don't screw anything up too badly.

Thanks!