Some of the older 360 games are now reduced in price Komp, some are defo worth getting
The very decent Perfect Dark Zero is only £8.99 at Argos...
Perfect Dark Zero for Xbox 360 Review - Xbox 360 Perfect Dark Zero Review
...you can play 2 player mode too
...also GRAW another 2player shooter only £19.99 at Argos...
Tom Clancy's Ghost Recon Advanced Warfighter for Xbox 360 - Tom Clancy's Ghost Recon Advanced Warfighter Xbox360 Game - Tom Clancy's Ghost Recon Advanced Warfighter Xbox 360 Video Game
some more I can recommend are
F.E.A.R
Condemned (excellent fps which can be bought for around a tenner at amazon used)
Condemned: Criminal Origins for Xbox 360 Review - Xbox 360 Condemned: Criminal Origins Review
Crackdown
Lost Planet
Gears Of War is awesome
loads really