Thought I'd make a quick list of worthy hopups with part No's. I'm guessing yours is the none worlds edition so wont have the big pulley conversion so that would be a good start.
Tamiya Centre One Way 18T 54051
Tamiya Center Pulley (18T) 53989
Tamiya Ball Diff Pulley (37T) x2 53988
Tamiya Reinforced Drive Belt x2 54140
Tamiya Low Friction Rod Guide Set 53979
Tamiya Stabilizer Set 53963
Tamiya 96T Spur Gear 51284
Hot Bodies Ceramic Diff Balls 3mm x2 67722
Most of the other upgrades for the 501x are for the 'bling' factor.