For P5/6 i would suggest to get al least 3GB RAM, so Poser can use the whole 2GB XP allows for each process. A celeron is also not good for rendering since the mathprocessor is missing. A regular P4 or Athlon 64 would be a better choice. For P4 the 5xx series is ok P6 does not make use of Hyperthreading and XP64. For the future a 6xx or 8xx would be better.