A Radeon HD 6670 will run fine on a 300w PSU from Dell. However, the caveat is the HD 6670 is a PCI-e 2.1 card. Your motherboard has either a PCI-e 1.0 or 1.1 slot.
There have been some people who reported compatibility issues when installing a PCI-e 2.1 card in a PCI-e 1.0/1.1 slot. The chances of having a problem is low, but it does exist.