Ok 400$ is very good now you have a ton of options.
So what you can do is get a good bike or get a medium good bike and as things break over time you can get better parts with the money you have left over and build up which is what i did.
Basically i started with a 300$ eastern traildigger 07 and over time the back rim broke so i got a real nice rim. Some kid bent my bars and i hated my forks so i got new ones. Then later i got a very good frame and transfered the cranks and everything over. This was a long process and gave me some room to build on my tricks considering i didnt have to worry as much about something messing up because i had money left.
So we can get you a cheaper bike like i did or you can get a bike that will last a good while and still transfer over to a new frame later.
Im just giving you choices but I would probably go with the better bike.
This bike would fit in between one to build up and a very good bike because of the sealed rear rim
http://www.danscomp.com/101667.php?cat=BIKES also the expensive of their bikes like the liberty and transition or whip i looked at some of their other completes but i was pretty satisfied with what i was seeing. Obviousley you will get a better bike from kink for the more you spend.
But i also have an even better deal for you right now for this month if you can buy a bike this month is the best time to buy. If you get one of the kink bikes from dans and you use the promo code 39d1 you get 15% off. So if you got the 429$ kink roc bike you would actually be paying 390$ after shipping and handling.
But the kink gap and higher are very good bikes for begginers.
Hope i helped and if you have any questions just ask.