Which credit card companies are likely to sue?

Long story short - got laid off a year and a half ago and stopped paying credit cards then. Suddenly I am getting letters in the mail from creditors from a few credit card companies (actually the creditors they hired). One is for Capital One that I am going to pay because I know they tend to sue a lot. The other is from HSBC for a balance of 400 bucks but they will take 200. HSBC hired Redline Recovery and they said if I don't respond in 30 days they mail a judgement to mean - meaning they will sue me for 400 bucks? The other one is from Citi Cards and the company they hired to collect from me (can't remember) but it also says the same thing. So exactly what does this - "you have 30 days to dispute the validity of this debt or we will send you a judgement" mean? Am I to take these serious adn start worrying?