Good choices there, IMO the 6600GT is still a very good choice as it can play current games and doens't cost a ton.
Try to get a 1GB dual channel kit for the memory, your overall platform will benefit. 2 gb of mem is even better, it helps with minimum frame rates which is what you'll notice, as well as application switching. I would take 2 gb cas3 over 512 or 1gb cas2.
The Antec P160W is a nice case with 2x120 mm fans, I like mine, although cabling was a little more difficult than some might care for.
You might take a look at the SLK3000B, it's cheap and roomy, and looks pretty ok...
PSU: Get a quality psu, max wattage ratings are bunk with most but the top manufactuteres anyways. Look at efficiency ratings and reviews, toms hardware has done some pretty extensive testing. Easiest cure: buy brand name. A good 400w should cover your needs. I am using the Enermax 400W Liberty for my current rig.