Welcome to the site.
I always start out with a good set of nerfs on all my quads. I am running the Houser Bounce Nerfs and they are amazing. They are a bit pricey but they really cut down on fatigue on the long rides. Then I add a steering damper. CCP makes a really nice one for the 700XX. This will cut out most of the bump steer and is adjustable depending on the terrain you are on. The next thing is the stock tires are near the worst things I have ever riden on and the stock wheels are kinda weak. The big problem is that the 11" rear wheels are only made for the 700XX and there are very few options for tires, Maxis RAZR & Kenda Kutters. I am running the Kenda's. I have upgraded my wheels to OMF beadlocks. They are pricey but they are the best wheel I have ever had. You can also change to a 10" rear wheel which will give you way more tire options but you will either need wheel adapters or a different offset wheel making the quad wider. So if you will be riding anything tight you may want to stay with the 11" wheels instead. As for the front tires most of us are running 22" or 23" tires. If you are running nerf bars you may want the 22" tires to be sure they clear. This will help with the caster a bit and make the quad a bit easier to turn. I am saving up now to get the EHS big 3 next. This includes the EHS air box lid, Barkers full exhaust system and EHS controller. Many people on the site run it and love it. If you are running the big 3 you may or may not want to change out the front sprocket. Oh and dont forget a good air filter.
2008 700XX - OMF Beadlocks w/Billet Centers & Reinforcement Rings, Kenda Kutters, CCP Steering Stabilizer, ASV F3 Levers, ODI Rouge lock-on grips, Houser Bounce Nerf Bars, Pro Armor skid pad, UNI Filter
2007 400EX - CCP Steering Stabilizer, GT Thunder 450R Shocks, 450R Rear Brake Conversion, YFZ450 Front Brake Conversion, ProTaper Bars, Rath Racing Nerf Bars, Alba Elite Bumper, ASV F3 Levers, ODI Rouge lock-on grips, UNI Filter, Kenda Kutters, Pro Armor skid pad, Armadillo Swing Arm skid pad