Is your RAM running at CAS 2 or CAS 3? Also, is that one stick of RAM or more than one. If it is more than one, enable SDRAM Bank Interleave in the BIOS. Are you able to raise the BUS speed any? That would help your times as it would speed up the entire system.
With your memory at CAS2 and everything else optimized, I would think you should be getting around 6-7 hours. Are you using the GUI screensaver or the command line client? If you are using the screensaver, make sure it it set to blank the screen. If you can, I would recommend using the command line client as it will generally cut about 10-15% or more off of your times compared to the screeensaver...