The small form factor Dells have an interesting CPU cooling setup that really should not be modified. Personally, I would just put the new GPU in and worry about additional cooling later. I wouldn't think your temps should be that bad. Most GPU's draw air in from the case and exhaust it out the back (there are exceptions). I think fitting it will be your biggest issue.
Nothing I see in those pictures is a fan header - the arrow points to a jumper on the mobo, not a power header of any nature. The three pin in the lower right of the second picture appears to be a propriety header (I can't tell what off hand) but won't fit a three pin fan.
If, for some reason you do need a new fan, best to purchase it from Dell.