This argument is a no brainer.
Modern PCs will always have better GFX as theyre always updating hardware. Xbox hardware is 6 years old now. I doubt a PC built on 6 year old hardware (which cost, say £200 6 years ago) would play any modern game 
My 2003 PC plays modern games still, it does however run with twin Geforce cards and have 8gb of paired corsair ram.......
My new PC cost me £380 with 21" HD screen, I have added an ATI card (£150) and overclocked the Quad processor.
I play graphic heavy online games at full speed with no issues, Consoles are best for gay point and shoot games like COD, hence I dont play them real man me......