Buying 360 Online or Lengthy Single-Player game...HELP

I am planning to buy another game, and I always buy games worth my money which means they must be either lengthy single player or have really good Online play

Recently Bought:
Cod 4 ( Amazing , still playing)
Halo 3 ( Amazing online )
GEOW ( Amazing online )

Do people still play R6: Vegas Online....saw a cheap offer so might buy that too

I was wondering if I should buy Mass Effect, I read somewhere it hsa over a 100 hours of play and it is a heck of lot of fun, or should I buy The ORange Box, not because of the Half Life Series, but because of Team Fortress. Is Team Fortress good? I have seen videos of it on the pc version and it looked like a type of game I would want to buy, so I was wondering if it was the same for the 360. Those are the two games on my list right now, but is there any other game out there either which is availble now or in December that I should buy? Either Lengthy Single-Player or Good Online Play.

Yea lots of questoin but I would appreaciate it if you guys answerd at least one of my questions, and some of you nice people might answer all :P

Orange box if you want online, as you have the other gooduns.

Or mass effect if you want single player.

Oh yea, I do have Oblivion in my collection

And I think the next game I might buy is the Orange Box because of TF 2, and the other 4 games as a side....should I do that?zaidpass

TF2 was weird. Like before the patch I would have to restart the system and I would experience a lot of lag. Maybe the lag fixed it but when I played it I restarted the Half-Life 2 story. The other four games are great you should definately check them out.

I bought The Orang Box, I was having lag with TF: 2 toobut its prolly because of my light-speed internet

Thanx guys