Low-profile single-slot width to be more precise. The PCI-E x16 slot is the second slot on the motherboard.
The Dell Inspiron 660s' PSU is a custom variation on the Flex ATX form factor. I highly doubt that you could even find a 500W-600W version. 400W is a possibility but not needed since you won't find a graphics card that would fit in that computer case that requires that amount of power.