I don't know that a direct comparison would be measurable. The standard PC benchmark is the PCmark05 software by Futuremark, I'm not aware that it an be used on the 360.
I guess you could look at the components but again the 360 has a multicore IBM CPU which is relatvely less well tested than the PC's AMD's and Intels.
Which really leaves only subjective comparisons of the gameplay. I saw Call of duty 2 on a HD monitor and it looked good, on the PC a smiliar spec to the one you mentioned, I think it is sharper picture and smoother gamplay.
A small difference though with that particular game at face value.