Well I am going to get into Cardiff at about 3pm on that day. I would suggest heading into the Bay until people get there, there are a few nice pubs over there. The Packet is a nice traditional pub and Terra Nova is nice to sit and overlook the Bay if it is a nice day, you can get food here too. The Wharf Bar is alright for drinking outside as well. Great place to be on a sunny afternoon.
Then get a cheap taxi into town. The main drag in Cardiff is St Mary"s Street really, we can do a pretty good pub crawl. Some good pubs that basically run from the train station to the other end of Cardiff include:-
The Great Western
The Prince of Wales (Wetherspoons) - Good for food again if people need it
The Cottage
The Goat Major (Traditional Pub, good on football days)
The Irish Bar (Good Guniness)
For Wetherspoons fans you also have The Gatekeeper and Central Bar on the trail and The Queens Vaults as Paulie suggested. There is O"Neils as well and The Owain Glyndwr.
Then you have Lloyds bar and Varsity for when things start to get silly and the shots are folowing.