From what I know, you'll never get near 400 hp with a NA without dumping in a V8 or with serious amount of cash.

A NA can be moded up to 270 or higher without breaking the bank but I'm with others on here, I'd suggest starting with an SRT or getting a different car. I think for about a grand you could add another 30 to 40 hp with some Needswings dual CAI and a tune. Certainly can get more with other mods but then you start getting into more money and dimenishing hp benefits......

Anyway, good luck. The rice rockets can be tough, but I still prefer my XFire for many other reasons.