Any reason the case I chose isn't great? The reviews seemed decent, and the smaller size seems alright given the lack of graphics card...
Also, is 380W enough for most tasks minus gaming?
A couple of reasons I chose the Antec 300: 1) I've seen it, worked with it and like it (I don't know the other case), 2) It's even more highly rated and 3) you said you expect it will be the only thing you use in the future and being ATX it'll allow for more options -- like an ATX motherboard with 2 PCIe slots -- if you want to turn it into a gamer.
BTW, the dimensions on the Antec 300 are only 2" taller, 2" deeper and less that 1" wider but you get a lot more flexibility.
That 380W PSU will definitely run what you have spec'd. You could even add a HD5670 GPU and still be fine. That is the PSU I have in my HTPC with that GPU.