Marc, I use Chris Warlod poly bushes:
964 / 993 Control Arm Bushings - Rennlist Discussion Forums
993 people are happy with them, Jeff from Rothsport suggested them also.
Probably ideal will be to have rubber (stock or Elefant) in back - rubber does absorb vibration, saves stress from chasis and steering rack. As for fronts - poly should be much better than rubber, see my other post on front rubber flex that causes toe out under braking.
PS I also ordered poly bushes for steering rack, but did not used them - fit was not good and I do not want to put extra stress on fragile looking steering rack mounting brackets. i did used 993 steering rack brace - it is on pics.
Oleg.