a. that cooler master is a mediocrely bad psu. yes, its compatible. in fact any atx is compatible. EXCEPT FOR ONE THING!
see the picture of the back? then look at the picture of the new psu. see that big switch on your psu? that will get in the way of mounting it in the dell case. you have to get one that is intended ass an oem replacement and has the big power socket in the right place and has no big switch in the back.