Epic Spell Wars is great fun, BUT I would say that you need three or four players to get the most out of it. Some of the cards target opponents in different directions etc. and you don't get that extra element with only two players.
If you enjoy Magic, you might want to have a look at Call of Cthulhu: The Card Game. Grab the base set and you'll get a good deal of head-to-head play time out of it before adding any of the expansions. Same thing for Android: Netrunner.
If you enjoy your co-operative play, grab a copy of Castle Panic and the Wizards Tower expansion. Also consider Elder Sign, Eldritch Horror and Ghost Stories -- though all are punishingly difficult to win.
Speaking of punishingly difficult, also think about picking up Space Hulk: Death Angel. It's a co-operative/solo card game that just LOVES to kick your ass, and it's pretty cheap.
I'd also recommend Ascension: Chronicle of the Godslayer or Storm of Souls for an intro into deckbuilding. It works really well for 2 or 3 players and makes for a very quick in-between game.
Oh, and CLAUSTROPHOBIA! For two players only, but man oh man... I'll recommend that any chance I can get.
If you like Love Letter, pick up a copy of Coup. It's from the folks who made The Resistance, but works with much fewer players, plays very quickly, and is really great fun with three or more people. And it's small (so goes to the pub easily) and cheap.
For head-to-head war gaming (on a more accessible scale), consider Battlelore: Second Edition and Mythic Battles. Both are fantastic.
In terms of Munchkin... well, they're pretty much all the same except for a re-skin, so if she loves Adventure Time then go ahead and grab that. It's a game that needs at least four players to be interesting in my opinion, however -- and you'll very quickly get fed up with it. I refuse to play it anymore because it just drags on FAR too long, and the wife and her friend always decide to go full-on "take that" when it comes to me, refusing any help, battering me down any chance they can get while ignoring anyone else who is obviously sprinting to victory. Just makes it a real chore to play.