If you could get access to RAM, CPU, and Northbridge voltages, RAM timings, and FSB:RAM ratio, you could probably push it farther, depending on cooling and how well Dell boards/your RAM Overclocks.
Northwoods are better than Prescotts. But watch out for SNDS (sudden Northwood Death syndrome) And I'm not kidding. One day our northwood will just die without showing any symptoms of dying. Just don't pump too many volts into it.