The newest wheezy-raspbian image updates the raspi-config program to allow preset overclock speeds.
Here is how I updated mine:
sudo apt-get update
sudo apt-get upgrade
Then run your "sudo raspi-config" and you will see the overclocking options. Some of these settings are aggressive and made my Raspi lock up and just misbehave. After experimenting a little, selecting "modest" setting with 300MHz core and 0 overvolt provided a good stable overclock for me. The "Medium" and "High" settings were causing a lot of crashes.
If you are running a wireless WIFI dongle, the update installs a setup application that can accessed from your desktop once you reboot. It is TONS easier to setup using this application rather than tweaking with /etc and /network files.
Hope this helps