RalliSport Challenge and RalliSport Challenge 2 are arguably the greatest rally racers ever made. They're fast and unrealistic, but just generally awesome. They're also both extremely gorgeous, they run at 60fps, and RSC2 seems to be some people's contender for most graphically impressive games on Xbox.

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Also in the racing game arena is Project Gotham Racing. I never personally fell all that deeply in love with it, but it's a very highly-regarded racer by a lot of people. The driving physics are satisfying, and it's a solid challenge.
PGR2 is supposedly good as well, but I haven't played it.

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One vehicular combat game that deserves a shot is Blood Wake, which is all about naval combat. The gameplay isn't the deepest ever (heheh...), but it can be pretty good fun, and it's fairly unique. Also, the water system is totally sweet: here's a video of me just sort of blowing things up with a big ship in an easy deathmatch game:

The sad thing about Blood Wake is that its sequel was cancelled. The game is good, but it did have a ton of room for expansion and improvement. Bleh.

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I haven't played Ninja Gaiden, but it seems to receive tons of praise.