Prey is just like Quake 4 and Doom 3. They're all pretty much the same game, with different models and textures, due to the fact that they use the same engine. From Doom 3, to Quake 4 and Prey, the requirements didn't change much at all and could still run fairly nicely on an Xbox.
Doom 3 ran without problems on the Xbox, Quake 4 and Prey only require slightly higher memory and processing speed, both of which could be made up with slightly more heavily compressed textures and lesser particle effects (watered down, yes, just like the original Doom 3 was for Xbox). Both games would still look pretty, and Xbox gamers could enjoy both of those titles just like Xbox 360 gamers.
I'm not trying to say that the Xbox 360 isn't powerful, I'm just saying that these games require almost nothing to run nicely. That's the power of heavily optimized and advanced game engines.
Links:
Doom 3:
http://www.doom3portal.com/info/systemreq.php
Quake 4:
http://forums.gamespace.net.au/showthread.php?t=6721
Prey:
http://www.prey.com/support/