I am currently using ATT's DSL.  I am limited to less than 3 Mbps connection speed, but the data itself is unlimited.

This is slow as shit when you have myself (online gaming, etc), my wife watching hulu/netflix on the TV and son/stepson running laptops and tablets watching TV/gaming, etc.

The local cable company has offering of 20 Mbps for less money.  But is limited to 250 GB of data each month.  

I would like to be able to tell if this is a reasonable limit.  I think it is, but I am not sure of our monthly usage.

Is there something that I can install on my laptop that can monitor the whole connection, not just the laptops.

No, you need to monitor activity at the router level. What type of router do you have - as it may already have that as a builtin feature?

Movies/netfix in HD gonna eat that pretty quickly

Some routers will do that. dd-wrt does.

ETA: I run a publicly-available connection, and I don't do 250gb/month. I use netflix too.

It is a little cisco router box that came with the DSL service.  I'll look at it when I get off work.  Maybe it has a counter on it.

Honestly unless you're really an outlier as far as the ISP goes, I don't think you'll have a problem. 250GB isn't really uncommon, and other ISPs are known to heavily throttle your connection if you're going over that anyway. A lot of people use Netflix and the like now, so I don't think that's quite as big a deal as you think. The people they're really trying to shut down are the hardcore bittorrent and filesharing folks.

Granted, I'm not pro-ISP in any way and caps suck in general. But like I say, if you're not an outlier, those rules probably won't get in your way.

There is no way that I could stay under that 250GB a month.  

Research
Sophos
PFSense
Untangle

All will give you usage stats and provide you with a bunch of security.

Buy this router http://www.amazon.com/RT-N66U-Dual-Band-Wireless-N900-Gigabit-Router/dp/B006QB1RPY/ref=sr_1_2?ie=UTF8&qid=1374695091&sr=8-2&keywords=rtn-66u

Then install Tomato firmware.  I use the shibby version, which is available here: http://tomato.groov.pl/?page_id=119

You are probably well under 250GB now.  Unless you are streaming 4 or 5 hours a day of TV and downloading a lot of other crap, you won't be hitting 250GB.