You need to establish a budget. Only then can we assist you in spending your money.
Easy stuff is better intake, carb, but that's gonna cost almost $1000 for new parts. Even a simple nitrous system will run you close to $1000 if you do it right, and you use all new parts.
If you get new heads ($1500), you're going to want to get a new intake and carb anyway, along with a cam. Now, we're up at about $3k, and we haven't bought a nitrous kit yet.
See how things can snowball on you?
The old saying "Speed costs money - how fast do you want to go?" is entirely applicable here.