You can do any DLC except Broken Steel from the getgo
So how does Fallout work with the DLC stuff installed? If I complete the storyline of the main game do the credits still roll? Do I have to beat it to make the DLC available, or can I just wander over to Pittsburgh or Anchorage at any time?
As demi says you can do them all whenever you want, as they're really seperate maps/storylines that don't relate to the Capitol Wasteland. Broken Steel quests are all after the main storyline is finished.
Not sure how it is at low levels though because I did mine all around level 25-30. You get some awesome gear though. 
MAN OPENER 
I was wondering what all those "mysterious" radio signals were, but I was too busy wandering around gawking at cool shit to find out. Sweet. I'll probably do some of that soon rather than later so that there's still some quests and exploring left to use all the jizztastic new loot for.
So far I've:
-Disarmed the bomb
-Did the Blood Ties quest (probably my least favorite quest so far because of all the trekking through the tunnels, which is the one annoying thing about this game)
-Meandered all over the place looting and slaying. Encountered a few groups of hitmen including a gang of bitch-made Regulators
-Let the ghouls into Tenpenny Tower and got myself a pimp-ass Ghoul's Mask
-Talked that hopeless distinguished mentally-challenged fellow out of writing her shitty book
-Got the satellite dish for Three Dog from the Museum of Technology and had him point me in the direction of Rivet City, where I am now
I know that the map is smaller than Oblivion, but the environment and atmosphere somehow make it feel a lot more "dense", if that makes sense. Not just that, but this is the first time in a long time I've played a game that managed to be so constantly unsettling. Not overtly scary horror thrills like in Silent Hill or RE, but a subtle eerie vibe. Like stumbling across a ghost town and seeing the strung up denizens blowing in the wind, or walking into an abandoned convenience store in the middle of nowhere and seeing a GODDAMN LIT CIGARETTE ON THE COUNTER WTF. Spooky shit, man. I always feel a little on edge when I'm ransacking a building, even if I
know I've cleared the place of baddies and nothing is popping up on my radar. I totally buy into the atmosphere of the game -- which is surprising, because for the most part I found the Elder Scroll games kinda dry (but still engrossing in their own right). I think this might even be better than Morrowind, but I haven't played that in forever so it's tough to say.
This was probably the perfect game to help reignite my interest in current-gen gaming. The technology totally lends to the atmosphere, and it's never pissed away on cheap frills; no pretentious narrative cutscenes that pander to distinguished mentally-challenged fellows, no look-but-can't-touch scenery, just immersion and pure awesome.
