Here are the system requirements from SM-it seems likeyours is in the ball park:

For Macintosh: Mac® OS X 10.4 or 10.5; 700 MHz G4 processor (Intel® Core™ Duo or 1 GHz G4 or faster recommended).

For Macintosh and Windows: 512 MB RAM (1 GB or more recommended); OpenGL enabled graphics card or chipset recommended (recent NVIDIA® GeForce® and ATI Radeon™ preferred); 24-bit color display, 1024x768 minimum resolution; 2 GB free hard disk space (4 GB recommended); Internet connection required for Content Paradise™; Adobe® Flash® Player 9; DVD-ROM drive (physical product only).

Could it be a graphics card issue?

I would open a ticket at SM...

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