That is quite a robust and good looking case for sure.

I forget the spec of your current laptop, but for GPU I would definitely got for the AMD 5870 if you can as it will give the system more graphical power and let you run all current games on a good high setting. It will also run rings around the 285M which is a middle of the range GPU. The equivalent nVidia chip to the 5870 would be a 480M which would cost a lot more and still not as good, so stick with the AMD 5870 if you can.

As for CPU. An i7 is much more than just some extra cores. Remember that the i7 has a much more advanced triple channel memory controller architecture so can run memory in tri-channel mode and supports faster DDR3 ram also. 3x 2GB of DDR3 in triple channel mode will give a huge performance increase over the i5's older dual channel setup. The i7 also uses QuickPath (QPI) which is currently much faster than any other FSB like system. Think of it as similar to AMD's Hypertransport but faster. i5's use the older DMI IO bus which is similar to that already used by the older Core2 chips.

However, to balance cost, an i5 CPU would still deliver enough CPU power if combined with an AMD 5870 GPU, so I would go with that configuration unless you can get an i7 setup for a good price and are willing to pay the extra for it.