for around $400 your choices are limited. how old are you anyway? if you're in HS you dont really need a lappy if you have a decent desktop....i just finished my junior year and i have 2 laptops, but thats besides the point. I say that if you're anywhere between 8th grade and senior year dont get one, wait till you graduate and grab yourself a nicer (~$1200) system for college.
there is the Compaq V2000Z which is the only computer (new) that i am aware of that fits your price range, otherwise you might wanna invest in a used IBM Thinkpad or something along the lines of that.
If you're not planning to hold a lot of music/movies/games on your laptop 40gb is more than enough, I have 60 and the most i ever used was 30 between the 3 OSes i ran at the time (i had winxp, mac os, and linux installed) Currently with only one os (linux) i am using about 10gb and thats including 2 games i have installed.
btw, by "learn about memory" he might have ment ram. you wanna get at least 512mb regardless of what you do, maybe even 1gb, and a good brand too (kingston, corsair, mushkin, etc)