Killzone 2 really is as good as they say, you can search for reviews on youtube to find out more, metal gear solid 4 is a must buy, Prototype is ok though its quite gory but it is a similar open world type of game. red faction: guerrilla is amazing as well. Little big planet has a lot of customizable features and is also quite fun. i recommend you searching for game reviews (particularly from IGN on youtube, they’re very helpful).

I recommend you wait for fifa 10 as a demo is bound to be coming out soon so you could try downloading them from the ps store. You might find other demos and games you might like there so I definitely recommend you having a look. Hope I helped!