Go with new vegas.
The original fallout is very, very dated. You can get the original fallout for free from gog or direct2drive? One of those sites has it for free.
EDIT
Gamefly has fallout free to members
http://www.gamefly.com/rent-games/Search/?kw=fallout&sec=GameFly&doSearch=
My opinion:
Fallout was a classic, largely kicking off the 'post-nuclear world' game design IIRC, and known for a dark sense of humor and interesting character traits.
Many do say it's dated now, though I think only in ways. It was never Crysis.
I think it's very worth it - but as others said you can get it for free.
Fallout 2 I've barely started, but it seems 'more of the same'. One claim to fame is the option to get married - and then make money selling your wife for sex.
My impression of Fallout is 'worth it', but not adding a lot to Fallout. It's the main reason to get the package.
Fallout tactics has fans, but I hated it. It seemed hard and had very little fun to me. I know, shouldn't it be fun with a squad armed with things like machine guns, but it wasn't.
So, that leaves Fallout 2 and the chance you will like the very tactical turn-based fallout tactics. They say Fallout and Fallout 2 are reasonably different than other Fallout games.
Worth $10 for that? Up to you, but get Fallout to try it, and if you love it, you'd like Fallout 2.