Hello Hidrolic,
XPS 8300 was shipped with the ATI Radeon HD 5870 which is 11 inches/28 centimeters in length and requires a 6 pin PCIe power connector. ASUS-GTX660-DC2O-2GD5 has a length of 10.2 inches and power supply requirement of 225W with a 6 pin PCIe power connector.
XPS 8300 has 460W power supply with a 6 pin PCIe power connector and hence the ASUS GTX 660 is compatible with it.
Please write back if you have any further queries.