A new steering servo is a good start. I use a Hiteck 645 mg servo, for about $40 your steering will be greatly improved.
Alot of guys also use the Hiteck 5645 mg which is a digital servo, a few more bucks but another very good choice
BTW the only other thing you will need is a servo saver, the traxxas one wont fit the hiteck servo, but a Kimbrough with the adapters are only about $6. No other linkage is needed