I have played video games most of my life casually here and there but have only been online for about 2 years. I had never played an RPG. I have mostly been what a lot of people call a frag head playing Cod 4, Cod WAW, MW2, Halo 3, and Halo Reach online. After playing FO3 i quickly realized why the fallout series was so talked about. I bought fallout 3 game of the year edition on xbox about 6 months ago and have turned most of my " frag head " friends onto it. Bought New Vegas yesterday. The Fallout series blows away all these games. I'm a late bloomer to these games but they are simply amazing.. Any recommendations on games worth playing after I finish New Vegas? I heard Oblivion was good but dont know anyone that has played it.

I liked oblivion, but to be honest I got bored with it pretty quickly. That's just me. I know a few people who have played it since it was released to now.

The TES games, especially Morrowind and Oblivion will probably appeal to you if you liked FO3 and FONV, in that they also have open worlds where you can get lost doing other things than the main quest. FO1 and 2 are great games (in some ways better than FO3, but probably not in the way you would like). They are definitely old-school (combat in them is turn-based, for example), and the graphics and UI are a bit dated. But you can get them cheap enough now, so it may be worth a shot.

If you want to try an MMO, Fallen Earth has some of the Fallout feel to it -- I played it for several months, and loved my time in it (I hit the level cap and then got bored while trying to level an alt). It's not a perfect game but you may like it as I did. It also has one of the best crafting systems I've seen in any MMO so far.

http://www.stalker-game.com/

http://www.1cpublishing.eu/game/planet-alcatraz/overview is close to original isometric Fallout 1/2

http://www.borderlandsthegame.com/ -sort of Fallout lite.

From bit other genere:

http://www.taleworlds.com/ -one of the best games I ever played! If you like middle ages at last little bit, you gona love it. Its also nearly perfect sandbox. Where else can you experience battle of 200 heavy armour clad knights in firsth person?

http://www.1cpublishing.eu/game/7-62/overview -sort of Jagged Alliance in 3D.

http://www.fortzombie.com/ -bit different apocalipse but nice little game othervise.

And then of course original Fallout 1 - 2 if you don't mind outdated graphics.

Well, not in the exact style of Fallout, but Dragon Age is amazing. It's also an RPG but not as open (not even close) as Fallout. I still loved it and is among my favorite RPG's.
Fallout however is a unique experience I have never had to feel until now. I'm also usually a frag nerd playing lots of COD, MoH Mafia GTA and so on. Fallout blew my mind.

Oblivion is very good, but I would recommend getting it on PC if you can. The mods make the gaming experience truly complete.
If you don't like Oblivion but don't mind bad graphics, you could give Morrowind a bash.

Well after New Vegas you could play the first 2 Fallout games.

The mass effect series are a great set of RPGs. Actually most bioware RPGs are superb.

Bethesda and Bioware RPGs are unrivaled mostly.

I would second "mount and blade warband" if you like fallout new vegas. but it's not as well done as vegas dialogue-wise, but the battles are pretty fun.
I've played oblivion briefly (about 20 hrs or something) and it's not too bad, i suppose i would say it's similar to the fallout games, just with no guns. The dialogue is terrible in oblivion. But it's not bad for a time sink if that's what you need.

I would also make a small mention to minecraft, it's also a sandbox, but in a more basic form.

Fallout 1 and 2.