Volume Rendering - Resolution & OutofMemory

Dear All,

I tried to render a volume graphics in my Unix workstation (512MB memory).
The best resolution I can get is 128x128x128 voxels (float data type). If I
try to use 256x256x256 voxels, the system gave me the "OutOfMemory"
Exception.

I execute my program as below:

% Java -mx384m myprogram     

Note: I did try with different memory number such as 448, 256 etc.

According to my calculation (float data is 4 bytes), memory needed should be
(256 x 256 x 256 x 4) = 64MB.

Can anyone help me in this context? No major programs are being executed at
the background of the workstation.

Thanks and have a good day for everyone.