I've got a ton of XBX racing games, so I'll throw my two cents down on a couple of titles:
Colin McRae Rally 3/04/2005: The series is hands down the best rally racing game available in the US. If you're not into some tuning, racing only against the clock, technical tracks and you need background music, this is not the series for you. I also must say it's got a pretty steep learning curve for a racing game. Top notch effort by Codemasters. The 04 version features Live scoreboards and the 2005 version has Live Racing.
IndyCar Series/IndyCar Series 2005: The name says it all, its Indy Car racing, just like you'd expect a NASCAR labled game to feature that series cars & tracks. Very touchy controls, realistic physics, more of a racing fan's game than an average gamer. However, it is another excellent Codemasters game. The 2005 edition features Live racing.
NASCAR Heat 2002: Basic NASCAR game, features the 2002 season, so some drivers and tracks have changes, but the game does feature all of the tracks, including Daytona. This one is normally found cheap and can be played in a system link configuration. Very accessible to anyone.
Project Gotham Racing 2 : I think this is the premier racing game on the platform at the current moment. Easily accessible but plenty of depth, lots of track variations, plenty of cars, and a great Live community still. It helps out that this is now a PH and can easily be found for