Dont get an Xconnect power supply, they are crap. It is watts that matter, unfortunetely, so pay attention to those. The Antec you have is great, stick with that.
In order to have Dual Channel you have to have to like sticks of RAM in the slots that are labeled Dual Channel on your motherboard. So you would have 2 512mb sticks in order to get dual channel. Check your mobo book to see what the ram slots are.
A case is a case fan, just make sure they fit the mounts on your case. The standards are 80mm and 120mm, with 80mm being the size of most of the mounst in most cases. Get an 80mm, and make sure you get a ball bearing and not a sleeve bearing, they last longer.
The 6600GT requires at least a 350W psu to run. Connected to that, it would run unstable as hell, but it would work. Get more than you need. It wont hurt anythign, it will give your peices and power requirements that they nneed, and will protect you for future upgrades. Dont cheap out on the psu, it is one of the only parts that has a direct impact on every single other parts in your system. All the peices are great thought.