U can always compare graphics quality between different platforms to a certain level but in the end it depends what games u wanna play!!
If u wanna play Crisis, u need a DX10 high end PC!!
If u wanna play Forza Motorsports 2 or Halo 3 or Gears Of War (although its comin for PC also), u need XBox 360!!
If u wanna play MotorStorm, Grand Turismo HD, God of War 3, Final Fantasy XIII etc, u need PS3!!
I am giving examples of mostly upcoming games! (Future proofing ur gaming rig)
Cost wise, consoles will always be cheaper than a high end gaming PC