Same problem here, I'v a GT240 and GeForce 8800 GTS:

Chipsetmodel: GeForce 8800 GTS
Type: GPU
Bus: PCIe
Sleuf: Slot-1
Breedte PCIe-baan: x16
VRAM (totaal): 320 MB
Fabrikant: NVIDIA (0x10de)
Apparaatcode: 0x0193
Revisiecode: 0x00a2
ROM-revisie: xx.xx.xx - internal
Beeldschermen:
Philips 200W:
Resolutie: 1680 x 1050 @ 60 Hz
Pixeldiepte: 32-bits kleur (ARGB8888)
Hoofdbeeldscherm: Ja
Synchrone weergave: Uit
Online: Ja
Rotatie: Ondersteund
Beeldschermconnector:
Status: Geen beeldscherm aangesloten

I've switch cards to try it but its definitely something in the cards, BIOS flash?
Strange that with the same install a 8800GT, 256MB works fine.
A 7300GT, 256 MB works fine

[EDIT]
Tried a lot of VBIOS's but all the same, no calibrate.
Used other install methods EmphireEFI and iBoot. No difference.
Tried 10.6.0-10.6.4. Nothing helped.
Different EFI strings, even customs EFI strings. Nothing changed, still no calibrate?

Hope someone can help us.