I've found a nice copy of Fallout New Vegas in one of my boxes. As i never used Steam since recently the key was still valid. So i activated the game and installed it. There were updates that i must install before i can play the game.
Looks like 3.7 GB...urggs...
Now my problem is that it shows 3.7 GB to install on the left side, but in the little game box it says 8 GB and the counter what i've installed till now. Do i install only the updates or the whole game again? And if the later can i stop that and how?

You always download the entire game from scratch, or only the updates if it was already installed from Steam before. That size is the result of the compressed download, and while it is downloading, the game is being decompressed and installed at the same time. Once the download is finished, you will see that it has been installed more than those 3.7 GB ;)

And no, you have to download the game first to update it. But it should use your current saves if you have them in your system. You can pause it and continue later, though.

I think New Vegas is Steamworks only, so you could install it through the disk & it would be attached to a Steam account?
As far as I know, once Steam downloads an update, you can't stop it. You can stop it in the future by going to New Vegas properties, going to updates tab and change it to whatever you want.

https://support.steampowered.com/kb_article.php?ref=5357-FSQM-0382

Do that. Once it's done, the game will download what's needed from the internet.

I did that to download Metro 2033 and Unreal Tournament 3 and only downloaded what was needed.

not working for me. it still tries to install the game again. and that means me...it shows a nice 3 days downloadtime for the 8GB (add that it would cost me if lucky around €20)

Me too.
Good news is a lot of the AAA titles end up in bargain bins after Christmas and in sales, so bagging a steam disc for a couple of bits can be worth it.
Also - Fallout New Vegas will drain hours from your life if you get into it. Had to uninstall or die.

It really is a shame that PC gaming has basically abandoned slow internet users, but I guess the forward march never stops.

I know in the parts of Eastern Europe I lived in they circumvented this by selling pirated copies on a disc.

You could also ask a buddy to burn you a LEGAL backup disc of the game made using Steam's built-in backup tools.

The real sad part is, that i would love to live in Eastern Europe cause they have faster con than me....and i
live in Western Germany a so called high tech country ROFL The good is...i can play the game on my X-360...the bad i can't play FPS (and FNV is that imo) with a controler...i need my keyboard and mouse....the really bad with all those sales and me buying and buying it takes soooooo long
to save enough for the prepayment to get a better con...sniff i am doomed I've only dl 1 of my now 146 games on Steam...and that i've finished already...and all the others are way too big for me to dl them atm.