I might be digging up a not-so-fresh theme, but I'd still comment, so the people would get the info they might need.
First, I'd strongly recommend going to the airport by train. Not only there are less chances to get late, but also it's safer. Don't forget that the licensed and metered taxis' drivers are not paid for speed the get you to wherever you need, so they aren't in any hurry. Therefore, they won't worry much if you will be late to your plane. They'd say traffic, speed limits, etc - not their fault.
Trains, including the Metro are a lot more faster and reliable, for tourist.
I mean, if there's a designated car with a driver that transports you around the city during the period of your trip, that is another thing, but still, if it's not a 7-10AM departure, I'd prefer the train.
And it is a good idea to arrive either in DME or in SVO 2 hours before the departure of the plane. Regardless of whether you have luggage or travel light, it's just better to have that time than not to. And it's Russia, so you never know when and how this time might prove handy.)