Millennium Stadium (Cardiff)
If going by car, then forget the official advice to use the Park and Ride unless you want to die of old age before you manage to get moving after the game. You'll be much better off getting there reasonably early and parking close to the city centre. Leave the M4 at junction 28 (or even earlier if the traffic's crawling) and follow the A48 and then the B4487. You should be able to park within a 20 minute walk of the ground (closer if it's Sunday).
Alan J's tips are: Cathedral Road is a nice area with it's own pubs and only 10 mins walk to the stadium. Lots of decent B & Bs. Try Lincoln House (02920 395558), Town House, Church's (Charlotte's mam's), The Beverley, The Cathedral, Courtfield (227701), Ferrier's (383413).
The ground is even closer to the city centre than St James' is, and so there are literally dozens of pubs within a few minutes of the ground.
Internet Ground Guide (some useful info on getting there by car, plus pubs,
Millennium Stadium (official site, but not a lot of use)
Net Cardiff (pubs, hotels, etc)