Depends on what sort of games you prefer, but I'd definitely pick up Uncharted 2 if you haven't played it already. It's got a great SP as well as an extremely fun, and reliable, multiplayer. It'll keep you entertained for a long time. Could also pick up the first Uncharted, also a great game. Both going for £14.49 each on PLAY.
Agree with all the above. Based on what you have i'd also say Dead Space is worth a look, along with Batman AA it was my favourite game of the last few years.
GTA IV is a class game,and Infamous is fun to play. Good games have been a bit thin on the ground for me recently so most of these titles should be on budget by now. I think your budget rules out COD Black Ops and F1 2010, the last 2 games i bought, but its not much of a loss. Okay they're not that bad.
Red Dead Redemption going for £23.99 at Amazon. Totally immersive and good fun but also great for the casual gamer. It doesn't take long to complete but there's plenty of other stuff to do if you want. Plus buy the time you've finished with it you will have saved up enough to by the brilliant DLC Red Dead Redemption Undead!