I may be able to offer a small glimmer of hope.
The dirty little secret about Dell's printers is they're all actually Lexmarks in different cases. You might, then, be able to get a Lexmark driver to recognize your Dell as one of its own. Unfortunately, the Lexmark that the 964 is apparently derived from, the x8350, does not offer Mac drivers.
However, you may be able to use one of the two following drivers:
For the Lexmark X9350
http://www.downloaddelivery.com/srfilecache/9300%20Series%20Web%20Installer.dmg
For the Lexmark X5450
http://www.downloaddelivery.com/srfilecache/5400%20Series%20Web%20Installer.dmg
No guarantees, but those drivers may help.
All else fails? I'm a fan of HP's printers.
Power Mac G5- Dual 1.8Ghz (Late 2004) Mac OS X (10.4.8) 2.5GB/160GB/SuperDrive/Radeon 9600XT/AirPort & BT