Well, there's obviously a problem with lack of perspective going on here. Perhaps there are too many here who started with the first two and then moved through properly.
(There will be a quick version at the bottom)
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The problem in question for the OP is getting his girlfriend to enjoy the first two instead of just 3/NV. Well the hard truth is she'll probably always like them more than you if you consider the first two the "good ones". As someone who started in Fallout with 3/NV I'll always enjoy them in a way that someone who started with the first two likely won't. For me, 3/NV are great games that I legitimately love. For those who started with FO1/2, 3/NV feels like a cheap watered down version of the games that captured their interest so long ago.
As for getting her to like the first two, you can't force it. There's such a thing as hype backlash. The more you hype it, the more you sing it's praises over 3/NV the more she's going to be disappointed when she first plays. You have to ease her into it, and let her take the first step on her own. Don't force the issue. Seriously, I can't emphasize this enough, DON'T FORCE IT. If she is forced to play then she isn't going to enjoy herself no matter WHAT you tell yourself. All you're going to do with that is sour the experience. The first two games are such that they deserve more than that. Being a rabid fanboy is going to get you nowhere.
To give an example? Me. The entire reason I ended up playing and loving FO2 is because a friend of mine happened to give it a shot and enjoyed it, saying that while the gameplay is obviously a bit primitive by today's standard, it's storytelling blew 3/NV out of the water. Being the kind of person who enjoys roleplaying my interest was piqued. I had already gathered the story from FO1/2 by that point from my surfing on the wiki. I already knew about the master, Vault 13, the Enclave, so I knew how the stories played out. But I still felt that perhaps I owed it to the series to give it a shot. A few months later I bought FO2, then held off on playing it due to the fact that I was in the last month of High School. So it just sat in my Steam library, installed, and unplayed before I finally got around to it. I loaded it up, and began playing.
Suffice it to say the whole Arroyo sequence left a bad taste in my mouth. The gameplay felt clunky even for a game as old as it was. The story came across as feeling generic and boring. I almost gave up on it right there. But I continued on due to a sense of respect towards 3/NV. If those games could hold my interest then the predecessors deserved a shot. So I kept going. It wasn't until I came across Modac in their time of peril that things started to get better for me. I helped them deal with the Slags peacefully and become a prosperous farming community. It was after this that finally, after coming VERY close to the deadline for the quest that a feeling washed over me. Relief. Not relief that I had finished and gotten the rewards but that I had saved the two groups from wiping each other out. I had finally had that moment that, without me noticing, drew me into the game with no means of escape. The game had somehow managed to convey more emotion to me through text boxes than Bethesda's plastic faces and speech, maybe because of the plastic faces. From that point on I was a true fan.
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The point is that you have to realize that there is now a significant portion of the fan base who never played the first two. We're not stupid for liking 3/NV, just new. Don't force the issue, don't leap down our throats with criticism, the only thing you're going to accomplish with that is to scare people away from a great experience. Just let your girlfriend do things at her own pace, she'll come around eventually. Maybe later than sooner, but if she really starts to get into the mythos then she'll want to experience the originals for herself, even if it's just for the novelty of it.