330 or 350 kit, it'll run with, and beat a stock 400ex. Get an exhaust too.
If you really want to go faster, get new shocks. It won't be faster in drags and such, but you'll be able to ride trails and track a crap load faster.
2004 Yellow 300ex
- HMF Slip on pc/d black chrome, K&N air filter, Dynojet, DG nerfs and bumper, AC grab bar, red shockwears, 21" front and 20" rear Razrs, Works Triples, Burgard +3's, Tusk extended rear hubs, JT 14t front sprocket, Pro Design tether kill switch, Maier 250r hood, N-Style 400ex graphics and seat cover, Renthal CR high handlebars, 50/50 Grips, MSR gripper levers, UM swinger skidplate, Tusk p-brake blockoff...
04 Yellow Honda CB600F4 (599) Street Bike
D&D polished s/o w/ midpipe, Renthal Ultra-Lows, Frame sliders, K&D signals, Keiti tank pad, and wheel stripes, gettin 55+ MPG!...
98 Dodge Durango 4wd
- 31x12.5R15 Pro-Comp Mud-Terrains on Mickey Thompson 15x8 Classic II's, Flowmaster dual exhaust, a couple of Pro-Comp Stainless 130w 6" spotlights, Cobra CB, JVC Head Unit, 2 Kicker 10" Subs, Lanzar Heritage amp, SPL Capacitor, Polk Audio 6.5's & 6x9's, and 2 sets of Pioneer 6.5's, All hooked up with Stinger wiring...
91 Maroon Chevy Silverado ext. cab, 2wd, 350 - Gone but not forgotten...