You will need a whole new motherboard for the i7 and you will basically be building your own pc. This is why we rec. building your own. I personally dont think you will see to much of a bottleneck, the quadcore should be sufficient running at 2.5ghz with ur gtx280, needless to say i think if your were to push it up to say 3GHz you would see a decent increase.
I don't think you will bottleneck, because with such a good GPU, you'd need something really low-resolution to actually bottleneck (Low resolutions are more CPU Consuming). (I'm talking like below 1440x900 or something)