Glad you've been talked into Sossusvlei - it is a very special place. I visited it often while working in Namibia a few years back. You should have it mostly to yourself in winter. Make sure you talk your guide into a pre-sunrise departure for Sossusvlei. All of the lodging options, both camping and Sossusvlei Lodge, are located at Sesriem, which is 60 kms from Sossusvlei proper. The road is paved all the way except the last 4kms, which is soft sand (4wd a must!)Sunrise amongst the dunes is spectacular. The soft color changes and shadows will make your camera sing! And fortunately, it's one of those rare places where the pictures actually do the place justice. Dead Vlei is directly across the valley from Sossusvlei - it's about a 30 minute hike in but well worth it. The dead trees are 150 years old - with the apricot colored dunes as a backdrop it makes for some great shots.

One last note: while in Windhoek, don't miss Joe's Beerhouse. I know, I know, it's a very touristy place, but it's so full of Namibian kitsch it's well worth it. Great beer, food (if your a carnivore) and atmosphere.