'mayer' is fairly correct. When the using a mac with the GeForce 9400M GPU installed according to Apple:
''"The NVIDIA GeForce 9400M allocates a base amount of 256 MB for video and boot processes at startup. For example, a MacBook Air (Late 2008) with 2 GB of RAM installed has 1.7 GB of memory available to Mac OS X and applications (2048-256=1792). Mac mini (Early 2009) computers with 1GB of memory share 128MB of main memory with the NVIDIA GeForce 9400M."''
We have an A1283 (Mac Mini Early 2009) with 2GB of RAM installed. The video portion in the Graphics/Displays portion of System Profiler shows 256MB of VRAM allocated after startup.
see "About integrated video on Intel-based Macs" at:
[http://support.apple.com/kb/HT3246|About integrated video on Intel-based Macs]