I logged in after the 4.0 update and my class mission is gone from the missions tab. I'm only a lvl 33 and cant complete my class mission cause its no longer in the missions tab. Anyone else having this issue?

Im a computer moron. I know the very VERY basics of them. I play the game on a Dell Inspirion laptop, I know not the best for this game, but its all i have at the moment. I connect it to my LED tv so atleast it looks ok. I have the graphics on very low because of the lag. Please PLEASE in english what is the best computer i can buy that will support this game at its highest with no lag? Not concerned about an awesome monitor since i use my tv, just the best tower to make this experience its best. Again, im a noob at the computer thing. Thank you and please help!!!!