Basically, while silver_oranges summed it up nicely, there's far more to do than a night out in the dingy heckhole that is Metros. It all depends on your music taste really - if you're one of those types who thinks dance and anything lacking a guitar is crap, then I suggest investing in a gasmask and a miners lamp for any Wednesday night in Metros.
Otherwise, you've got a whole world of possibilities, and something for every day of the week near enough:
There's Oceana on Mondays, which is dead cheap; Vodka Revs on Tuesdays and Wednesdays; Walkabout or C-Y-N-T at the Welsh Club on Thursdays; and the weekend is your oyster with everywhere in town being at non-student prices.
Silver_Oranges is also right that you can walk Cardiff at stupid o clock, and nobody will do anything. Sure, there are stories of muggings and things, but they're so rare. I've done the walk back from town out to Gabalfa at 4am, and nobody has hassled me. I've walked through Cathays at 1am past a gang of local boys who on any other occasion would be labelled as knife-wielding hoodies by The Daily Mail, and nothing came of it. It's just so laid back here.
If you've already got a job, find out if that place is in Cardiff and see if you can get transferred. As much as everyone comes to Uni with an "I'd rather not have a job, and concentrate on my work" attitude, you usually either end up needing a job, or generally concentrate so little on your work that you may as well have had a job to pass the boring hours on facebook a little more productively.
And finally, you don't need to worry about Atrium lacking an SU. If you're that desperate for a drink and a chat, invite your mates out to the Vulcan. It's right across the road. Up where I am, the nearest pub is the Blackweir and that's quite studenty during the semester, so I'd assume the Vulcan is a bit like that. Never been there myself, but would like to go one day, since it's all historical and whatnot.
Best thing to do when you get to Cardiff - introduce yourself to your flat, have a day getting to know each other, and then just choose a place to go on the first night of Freshers'. Sorted.