I've got a 80Gb one, but I wish I'd got the 40Gb, which was cheaper and had two bundled in games.. 40Gb will be enough for you, unless you want to put your mp3 collection and some movies on it. Which you won't
Get the 40Gb one.
About backwards compatiblity: not all the 40Gb ones are PS2 compatible. The last batch were, but I don't think the current ones are. As far as I know, none of the current batch of PS3s are PS2 compatible.
And yes, NTSC is American. But there is also NTSC/J, which is Japanese.