I have a dell optiplex 745 sff I would like to know what graphic card is best (low profile) I currently have asus AMD readon r7 240 2gb low profile edition with 4gb ram and will be upgrading my CPU to a core 2 quad Q6600
Which R7-240 do you have now? The optiplex 745 small form factor demands a single slot card. If that is what you need, you are at the top of the food chain now. Plus, your stock PSU is a limiting factor.
How would I know if I can fit a dual slot in ?? Because I have two plates I can remove from the case where HDMI connection is ??
Unfortunately, those other expansion slot covers aren't going to matter. Look at this picture of your 745 SFF: http://en.community.dell.com/cfs-file/__key/communityserver-discussions-components-files/3515/SLOT.JPG
A 2-slot wide card will extend upwards in the picture, not downwards. Dell probably had the m/b designed that way to deter anyone from trying to add a card that was too much for the tiny PSU they provide.