Cardiff is very accessible from any of the major hotels. There are quite a few right in the city centre, and most areas are easily walkable from there. If you go down to the Bay, you'll probably end up wanting to catch a train in to town, but the bay area is still nice and the train ride is only a few minutes (It is walkable from the Bay into the city centre, but I wouldn't do it personally). There are a string of cheaper B&Bs on Newport Road, but those will be a good bit of walking or a bus ride in. I can't remember names of specific hotels and such, but I know Cardiff quite well (having lived there for three years). Feel free to PM me or repost if you have any questions or want to know about a specific place.
Do enjoy, I have loved Cardiff. Don't if you have some time, the tour of Cardiff Castle is a kick, and if you have a day, I love going to St. Fagans, a living museum run by the National Museum of Wales. (It's also free!)
