I apologize. It's the xps 1640

It's just about impossible to point you to that, without knowing exactly which model you're asking about.

any documentation that can back that up?

If the system uses a Core 2, the memory is DDR-2 and either 667 or 800 MHz depending on the CPU.

If it's a Core i system, which uses DDR-3, it's either 800 MHz or 1066 (again, depending on the CPU).

The tech specs on the PM45 chipset are here:

http://www.intel.com/Assets/PDF/datasheet/320122.pdf

The Studio XPS 16 was/is built both ways - depends on your CPU and RAM type as to which mainboard you get (DDR-2 or -3;  they're mutually exclusive).

Depends on the system model -- only the newer Core i systems will run memory at 1066 MHz.  If you have an older Core 2 system (even one with a 1066 MHz FSB) it will not.