First of all, the explanation given by clutchc is complete and very good.
All HD5450 I've seen are low profile so they will work for you, regardless of your PC's case. Just be sure to have all brackets. The card comes with the big (full height) bracket mounted and has 2 small (half-height) brackets - one for the DVI/HDMI port(s) and one for the VGA D-Sub connector.