Bearings yes, it'll make it much nicer to drive and the motor/geartrain will last a little longer. Alloy parts...I'd say no, the extra strength from the alloy arms etc will simply move the weak points onto the chassis, arms are pence to replace, chassis are pounds! And I can't say I've noticed any difference with alloy bits, except metal gears and hex's (and half shafts if yours are plastic?)
What's the plan, just a bit of fun or to get more serious? I used to race a Team Associated TC4 a few years ago...couldn't afford to keep it up though

I'd say your next buy should be a LiPo battery and charger, sooooo much better than NiMh or NiCd!