The low end 15" (the 2.0GHz quad) is designed for people who don't play games. It has an amazing CPU, but is lacking in graphics power.
The high end 15" (the 2.2GHz quad) is designed for people who enjoy playing intensive games when they aren't working. It has a graphics chip that can easily handle almost any game.
If you care about graphics capability, the 2.2GHz quad with 6750M is what you should buy. It is much faster than the old 330M.
The GPU on the low end, the 6470M or whatever it is, is very weak.