For many games the i5 950 and the i7 870 will be just about equal, although the i7 does have an edge in all benchmarks and will be faster in video rendering.
While a high end video card may be bottlenecked by those CPUs depending on the settings and games, it will still be a huge improvement on the stock graphics that would have come with an ACER system.
Keep in mind you need to check your exact motherboard to see if it will support the CPU you want, not just if it has the same socket.
And if you are just now upgrading to a fast video card from stock and the CPU, you will need to upgrade the power supply.