Yeah be careful of a lot of your earlier now "budget" titles.

Games like Prey, Quake IV are so laggy they are horrible. Last I tried the Darkness is not even playable! I hear FEAR is pretty good. Last I tried it Farcry is decent. Dunno much about Timeshift I'm hearing mixed, I fired the game up and my first impressions was not impressed.

Something else I though was decent but didn't do too much of was Saint's Row.

Call of Duty 3 (especially now for its price), is a very addictive online play even MOH Airborne is fun (although that one can be quite glitchy).

But like others have mentioned the best game out there right now, the must have is COD4.