Its all budget based and what type of riding your lookin to do. But if you got on a 450 and liked it so much, sad to say that your warrior will never cut it for you unless you go extreme. I've ridden 400 thru 700cc quads, yet I will always be happy to hop back on the old trusty warrior. If you have that type of crazy mindset, then have fun modding your quad, its all part of the fun!
Best for the cheap Is simple, exhaust, airbox and filter, rejet. For a bit more money; Add to the above - 1 over bore and 390 cam with thinner gasket kit and HD springs. Even more money added to the above - Switch cam to 430 or bigger (or keep the trusty 390) switch out carbs, +1 intake, P&P, stroker crank. Balls to the wall is the 446 kit.
All this info has its own post. Take some time to look around and read some of the more popular posts.
Enjoy!