I have a Dell e520, Pentium D (64-bit Dual Core) 2.8 GHz, 2 GB DDR2 RAM, with Windows XP. There is an option in the BIOS to disable the dual core part of the processor. Are there any pros and cons to doing this?

Keep it enabled. There's no point in turning a dual-core processor into a single-core processor. If a legacy program doesn't take advantage of SMP, then it will only use one core. 3.2GHz on S939--E.T. no need phone home. Opty 185 @ 3.2GHz Dual 8800GTS-640, both flashed to 625/1458/1950 4GB PC3200 HD-DVD/Blu-Ray combo A8N32-SLI Deluxe Big Typhoon HSF

You save power and less heat is generated but you loose performance. I think its basically pointless to turn off one core linuxant

Turning off a core for notebook processors isn't pointless though becuase it helps save on battery life and Core 2 Duo mobility processors do this automaticly when the other core isn't needed. AMD dual core mobile processors don't have this feature yet.

if there is a program that wont run on both cores or only needs one core then goto the task manager and set the afinity to just one processor. Right click on a process and goto set afinity. Alternatevely, you could disable the multi-processor HAL in the device manager and change the driver for ACPI to single processor PC and back to multi processor PC when required.