Fogold,
The effect of case material on processor temp is negligible. The amount of air and the ambient temperature of same is what matters. If your XP processor overheated, I'd look for an improperly seated heat sink, inoperable heat sink fan, too much or no thermal paste.
The XP 2500+ computers I've built run @ 40 idle, 45 full load in an Antec steel case, with the stock HSF and they're running @ 2.1 GHz (200 x 10.5).