If you click on the network icon in your panel, and select Edit, you get a dialog where you can select your active network connection. Select this and click Edit, then click the IPV4 tab, then you will see a text box where you can set DNS servers as a coma separated list.
I have experimented with Google's DNS servers but have not seen any noticeable difference at my location. However I have added the OpenDNS servers because I find I need these to make GoogleEarth reliable.
Desktop: Dell, Intel Core 2 Duo E7500 @ 2.93Ghz, 2GiB RAM, Linux Mint 13 Maya.
Net-book: SAMSUNG N148-Plus, 2x Intel Atom CPU N450 @1.66Ghz, 1GB RAM, lubuntu 12.04.