probably do 4th edition, its the most recent, and its easiest to learn, and most vercitile (You can use the streemlined rules to just go aroud killing things, or to get thru the combat encounters faster and do more roleplay). If your just looking for a good time, all you really need is the players handbook, you will have to make up your oun monsters and come up with your rewards based on winging it, but you will save about 100$. If its just you and your girlfreind its going to be lame, you need at least one Dungeonmaster, and 2(Absolute minimum in my opinion) players. Your also going to need dice.
If you want to get more into it, get lots of dice, and the players handbook, monster manual, and Dungeon masters guide. Now, if your going to have probloms geting people on board because they think it is geeky (Sadly one of the bigest probloms) Do 3.5 or3. Because 4th editions illustrations are a lot more childish. I just ignore the pictures, the rulles are streemlined and easy to use (If your familiar with the D20sys). The D20 system seems really complicated at first, but its really not.
"A true king is never alone, his will is equal to the will of all of his followers."
-Alexander the Great, kind of