Creston is basically right. I'll echo what he says, including that bit about the limited CPU on the XBox.
The thing is that you need software to run on that hardware. PC Hardware is clearly much more advanced, with faster clocks, more/faster memory, more rendering features, 32-bit floating point vertex and fragment programs, etc. But its not really being used in games yet.
DOOM3 is getting close, and some codepaths in the game use pixel fragment programs, I think, for optimization purposes, but even that doesn't really use everything a GeforceFX can offer. Only the next generation will truly show what is possible.
But even after all this, I don't think there is anything graphically like DOOM3 on the Xbox. The PC will get this game first, then an XBox port will happen. But don't necessarily expect it to look the same. I figure they'll have to cut the PC game down somewhat to guarantee a steady framerate on the XBox.