we need more information to properly answer your question
1. there is a label with the complete model of your 660s, usually on the case top or rear or side look around,
(i.e. Dell Inspiron i660s-3848BK for example) we need this to figure out dimensions. to confirm if card fits in case or not.
2. most dell power supplies are not standard sizes, nor very powerful (depending on case format it ranges from 220-420w) which depending on the video card can be an issue of not having the appropriate power connectors to supply power to your new GPU, and depending on the case format you have can be almost impossible to find a replacement to upgrade this PSU.
3. case and point, 1060 3g or 6g require required a recommended 400w of PSu and 1...