For $US35, you can't expect too much out of the tiny 700MHz Raspberry Pi computer. However, according to the official site, you can now overclock the little processer up to 1GHz without voiding your warranty.
The overclock method has been built into the newest version of the Raspbian operating system. Once you download the new version of Raspbian, you will need to turn on turbo mode. In Terminal, type:
Select "overclock" and reboot your Raspberry Pi. This enables a dynamic overclocking that will automatically drop back down to 700MHz in case the Raspberry Pi gets too hot. If you have any troubles with overclocking or updating your Raspberry Pi, head over to the official forum thread for detailed instructions.
Introducing turbo mode: up to 50% more performance for free [Raspberry Pi]