Thank you very much for all the support. I have finally solved reinstalling MacOSX from a Combo Update. I think ity was a corrupted driver. Now this is my system configuration (as was time ago)
ATI Radeon X600 Pro:
Modello Chipset: ATY,RV370
Tipo: Monitor
Bus: PCI
Slot: PCI-E
VRAM (totale): 128 MB
Fornitore: ATI (0x1002)
ID dispositivo: 0x5b62
ID revisione: 0x0000
Revisione ROM: 113-xxxxx-113
Monitor:
iMac:
Tipo di monitor: LCD
Risoluzione: 1440 x 900
Profondità: Colore 32-bit
Integrato: Sì
Core Image: Supportato
Monitor principale: Sì
Mirror: Spento
Online: Sì
Quartz Extreme: Supportato
Monitor:
Stato: Nessun monitor collegato
Thank you again