well, there some things to keep in mind when comparing the two.
The X-box only has to run games in 640x480 (or what ever the tv's resolution is) and maintain 30 fps if that. The xbox also has a simpler os, so there are fewer layers of software and fewer programs running at the same time. so the xbos is more efficient and doesnt have to be as powerful.
The PC has multiple layers of software including the bios, system software, video dirvers and then the game itself, plus the game has to be written for different types of hardware and software thus decreasing the efficiency at which the hardware can run the game. With optimizations pc hardware will always be faster (and more useful) when it comes to running games.
I still think it as amazing that such perfromance can be gotten from a 733mhz celeron cpu with what is basically an nv25 (i think) GPU.
But getting more technical, console hardware can be incedibly optimized. I mean the ps2 has the 200mhz emotion engine and even though it is based on a .25 micron process, it still manages to compete with pc's and other consoles. i think i read that the emotion engine uses 32 megs RDram and has 48GB/sec bandwith with the CPU/GPU and can perform 10 instruction in paralell or something crazy like that. Well, I hope that answers some questions.