Hello louise,
If you go to the archives and want to look at records in the search room you will need to take ID and proof of address so that they can issue you with a readers card. Each time you attend they will simply scan your card and you can enter the search room. In the reception area they have a number of PC's so that you can use if you do not want to enter the search room, on these you can access Parish records as well as Ancestry and Find my past. Most of the parish records are digitised so you can save the images onto a memory stick in PDF format. The records that have not been digitised you can still search manually. The catholic records for Cardiff are typed transcripts of the original records and not exact images. The staff there are very good and extremely helpful if you get stuck.
Best Wishes,
Tallpaul.