So I picked both of these up a few days ago...
... and holy ****, XGRA is straight up the best racing game I've ever played. How the hell did this game slip under the radar? Faster than F-Zero GX, crazier than Wipeout, and more fun than Mario Kart. XGIII is alright, but XGRA just shines so much more.
This is a must play game, as in stop what you're doing and go buy it right now. It's on the PS2, Gamecube, and Xbox, and also probably on Steam, and it's one of those games that is so unknown that copies of it are like ten bucks max. And it's easily in the top ten games of it's console generation. That's how ****ing good it is.
Even if you hate racing games, try this one. Trust me. Once you break the sound barrier for the first time, you'll never want to touch another racing game ever again.