I agree Gears of War is much better than Halo.
Bioshock is great too.
I should warn you, since it seems you have restrictions in what you're allowed to get - both these games have minor swearing in them.
Fallout 3 is incredible - basically Oblivion with guns. Both games are awesome in their own right. Not to mention addictive.
Fable 2 is OK, it doesn't have as much playability as any of them above in my opinion, still a good game though.
GTA4 is alright also, though I got bored with it about half-way through.
The Orange Box (Half-life 2, Portal etc.) I haven't played this one yet but everyone raves about it, 5 games in one for £12 - can't go wrong really.
Beautiful Katamari is going cheap now, my friend loves this.
Hope this helps.