Read cd/dvd/and write cd dvd
It depends on what you want most
DVD writers will most likely write as fast as or faster than an older CDRW
Putting it on the HDD cable (may) cause issues
some RW drives like to be master and not slave.
Personally I would replace the CDRW with the DVDRW and leave in the DVD rom for fast read speeds.
That way you will also still have a spare ide connector if you want a 2nd hard disk.
This is just my opinion others will no doubt have their own way of doing this sort of thing.
You should also consider which type of DVDRW format you want to use before you go ahead and buy one.
The PCA reviews page will give you a good starting point for info