For a new system definitely the i7, you have no more upgrade options on LGA 775. If you look at the chart that warmon posted you will notice that when the CPU is the limit, in both cases the max FPS is significantly higher than you need, and once you increase resolution and detail the GPU starts limiting you. I would expect the QX6800 to cost you more than an i7, so an i7 would probably allow you to put more into a GPU where it really matters.