I don't know about Dell machines, but as a gereral rule for PCs the questions you ask can be answered as follows:
can i fit udma133 120gb7200rpm. hard drive? - yes
can i use my 40gb as slave even though this runs at5200rpm? - yes
do i have to fit the same make of memory in the 2 empty slots? - no
For memory, have a look at crucial.com/uk and tell it the type of machine you have. It will recommend the correct memory for you.