If you like the Batman universe then you really can't go wrong with both of them if your PC can handle them. Arkham Asylum is a bit of a tighter game, well knitted together and with a decent story arc to it. City is a bit more open and perhaps loses some of what makes Asylum so good, but is still a very well made game.
If you want something batshit mental and full of fun (but lacking somewhat in story) then grab Just Cause 2. Often finds itself in people's top games of this generation list, just because it's such an entertaining sandbox to play about in.
Something else pretty cheap at the moment, and more of an indie game is Bastion. Good fun action game, but also a pretty unique and interesting story that floats along with it. Personally loved the narrator that hangs about with you throughout the game, and an excellent soundtrack to go with it. Both are under ?5 at the moment, and well worth any gamer's time.
