The first upgrade should be the hpi threaded aluminum shocks. These work great as the handling change is night and day. Get different spings and shock oil and experiment. Next get some good foam tires if you want to race. I an assuming that the main reason to upgrade is to eventually race this car. If no intentions of racing, then upgrading this car can get pretty expensive as the list of upgrade parts is basically EVERY part on the car!!! Once you have gotten the shocks/foams (and learn how to setup your suspension) get the hpi super chassis. Its 3mm thick and 7075 alum which is much stronger than 6061. Stay away from any chassis that is NOT 7075 alum. Once you feel comfotable with this setup (recommend lots of practice especially changing shock springs and shock oil) get ALL the carbon suspension parts:
upper deck, f&r shock towers, steering knuckle/c-hubs and suspension arms. To fine tune and strengthen the suspension further, get titanium turnbuckles and put ball bearings in the steering arms. All these upgrades I consider stage one for a typical racing setup. Stage two would be to strengthen the drivetrain and finally stage 3 would be a high performance racing engine(hint: italian made!!!). Lots of stuff but only if you want to race or have deep pockets for modding. Hopefully we can keep this thread going for all the rs4-3ss car owners out there.