Well it depends what you want. If you want longer runtime get the bearing ($19.99). If you want to go faster get the stage 2 motors ($15.99). If you want better handling get the AWD ($14.99) or the springs upgrade ($7.99) or the tires ($7.99). If you want your car to look cool get a body kit ($12.99) or lights ($5.99). I know there are other upgrades out there but I am too lazy to list them all.
I'm for real... but then it's just my opinion... I went through several X-Mods... The radio and steering servo is just plain crap... If Radio Shack (ironic isn't it?) can't put out a decent radio control system...then they will forever be considered to be "TOYS" more than "hobby quality".