Dirt 2 is amazing, end of, great game, awesome races, AI leaves a little to be desired but pffft
Dirt 3 is almost as good, scenery looks better and it has a few more race options over dirt 2
I wouldn't bother with any of the recent Need for speed titles, they are all trash compared to other racing games available, ( yes i have most of the recent NFS titles

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also you could look up Race 07, it's a racing sim, not sure if it caters to dirt driving, but it's very cheap and tonnes of add-ons and various Mods make it pretty good
Trackmania 2^^ i don't have it, but it looks very good, if it's anything like the first game, it's very very arcadey but a great game for picking up and putting down
