The graphics card is going to be the most important part to get the best performance from games, and if you look at the benchmarks for the GPUs you can easily see the 5870 is performing a lot more powerfully than the nVidia cards. Also worth noting that the only cards coming close are all SLI which means 2 cards are needed so it isn't a realistic benchmark as you can also run 2 5870's together in crossfire and that would completely blow the nVidia cards away.

If you look down the list, the first stand alone card is a GTX 480M with a 3DMark06 score of 15019, whereas the single 5870 is achieving 19080. Not a perfect test of performance compare to real world game FPS scores, but it does show the increased performance.

Regarding CPUs. It is worth you doing some research on the different i5 and i7 CPUs as some of the lower cost i7's are actually really i5 technology rather than full i7, and are missing the triple channel memory support, so not worth the extra cost. Do try to get a quad core CPU if you can though as games are now being coded for multiple core CPUs so new games will benefit from more cores.