Link please. The classic "HR-03" (the one for 8800GTX) does not fit the 8800 GT. Thermalright has promised an "HR-03 GT" designed especially for the 8800GT, but I haven't seen it in online stores yet. It totally depends on which HR-03 you found.
 
Bad news, sorry. Read this:
http://www.thermalright.com/a_page/main_product_hr_03gt.html

We have a Support Question about compatibility of NEW 8800GT and Thermalright HR-03 PLUS. Is the cooler compatible? What about HR-03 Rev.A?
HR-03 PLUS’ mounting mechanism is not compatibly aligned with the mounting holes on the 8800GT so installation is not possible. As for HR-03 Rev. A, technically it can be installed on the 8800 GT but because the PWN IC on the back of the card is not covered by the HR-03 Rev. A, plus the fact that the heat generated by the new 8800 GT far surpasses the 7000-series, we would strongly recommend our new HR-03 GT

The one you found there is the Rev. A. As they say above, technically it can be installed but you should get the HR-03 GT instead. Keep looking...