Forza 3 is sweet. You can do engine swaps, add turbo/super chargers, swap rims and most of the changes you make are to the engine/brakes/suspension/transmission. I think you will like forza 3, but they don't really make games like nfs underground anymore. The most involved exterior mods on forza 3 are adjustable front bumper and spoiler and window tint. You can do roll cages and weight modification. As far as tuning is concerned this is one of the best games available.