Don't worry about the overclock. You can dial it back down just by entering bios and returning the multiplyer back to auto. But before you do that I would just download OCCT and check your temps while running your best whatevers. As long as it don't go over 62*C then there are no hazards to your equipment. The important thing is that it will run that fast if you want.
I'm glad you already have a case cause now I/you can spend more on the video card. This one is a little better Sapphire AMD ATi Radeon HD 6870 1024MB - 21179-00-40R for 195.11 so we are actually under budget!