im looking in to a 2003 altima se and i was wondering what the best way to increase the HP would be for around $2000-$3000

With out going turbo that would be to change headers, exhaust cat-back, add cold air intake (with fan insert), and reprogram ECU. Also you can change the cooling fans, but thats not necessary

thx so how much horses would that give me??
What if i did go turbo? how much would it cost and how much hp would it give?

that depends on the engine (for this engine I don't know of a turbo I could look and see if there is one out there. but for hp on the other upgrades it depends header type, exhaust size and ecu reprogramming. cold air intake 15-40 depending on type (manufacturer)