> a info i read some time ago that those minis were called "silent updates" by apple

The 1.5 GHz G4 Mac mini "silent upgrade" *always* has 64 MiB VRAM.

> there is on another forums people who has the mini at 1,5ghz but with 32 vram.

That's no "silent upgrade" 1.5 GHz G4 Mac mini then, but overclocked either by end user or by reseller company.