I click on "About This Mac" and it says:

Processor 1GHz PowerPC G4

Memory 640 MB DDR SDRAM



I click on "More Info..." and go to memory and it says:

DIMM0/Built-In 128MB Built-in Built-in
DIMM1/J31 512MB DDR SDRAM PC2700U-25330


So no, I don't think I do have 256 built in, I have 128.
But thanks