Hey guys 
 
I bought Fallout 3 GOTY when it was on sale on Steam and I was wondering when I should start ploughing into the DLC for it. Do I wait and finish the main questline or do I finish all the DLC beforehand? any help would be greatly appreciated.

Doesn't really matter, it's up to you. They don't spoil the main plot. You can however get some crazy overpowered armor and weapons though from the DLC, which you could then use in the main quest.

ha ha, cool. Is the Level Cap still 30 or did some of the later DLC raise that?

The level cap was on 20, but then raised to 30 within one of the DLC. As for where to begin, it doesn't really matter, but i can remember that i experienced a bit of hard times with "Point lookout" DLC. Or some parts in it at early levels. It could be that i wasn't really good at the game back then though.  
 
Edit: I meant point lookout, not the pit. The DLC where you need to take a boat to a new location.

Start anywhere except point lookout because point lookout is really difficult compared to the others.

Thanks for all your suggestions guys! really appreciate the help!

If you are a stealth-based character, the rewards, namely the armor, you get for completing Operation: Anchorage is a must-have.

Most of the DLCs require you to be a higher level, point lookout in particular, I'd wait till level 15-20 until trying that. I've also found Point Lookout to be the best DLC. The rest you can start, more or less, right away. If you're looking to jump into the DLC content straight away I'd say go with Operation Anchorage, you also get some pretty kick ass loot after you complete it. The Pitt is also an interesting DLC, it's not too difficult, but you have no where to place your stuff, and once you enter you can't leave until you complete it, you can however place the loot in a container and then retrieve it later on. Same thing with Mothership Zeta, I've also found it to be the least enjoyable of the 5 DLCs.  Haven't completed Broken Steel yet, which is why i started a new playthrough.
So in terms of difficulty (from lowest to highest):

1. Anchorage  
2. The Pitt
3. Broken Steel
4. Mothership Zeta
5. Point Lookout

1. Point Lookout
2. The Pitt
3. Broken Steel
4. Operation Anchorage
5. Mothership Zeta

I'd recommend the order I played them in:

1. The Pitt
2. Operation Achorage
3. Broken Steel
4. Point Lookout
5. Mothership Zeta

The first two you could really do before finishing the main story, but I'd definitely leave Point Lookout and Mothership Zeta for the very end.