Just note that not every hsf sold for LGA775 will work properly with your MB. Surprisingly, many "LGA775" hsfs, including some big names like the Zalman 9500, do NOT meet the Intel thermal spec for LGA775. This is because the hsf has 2 functions:
1) Cool the CPU. This is the main function and all hsfs cover this.
2) Cool the surrounding components on the MB. If you examine the OEM Intel hsf fan and other "flower"-style hsfs such as the Zalman 7700, Thermaltake Golden/Blue Orb II, you'll notice that they blow air *down onto the MB* in ALL directions, air-cooling nearby components such as the PWM area and northbridge. Many (most?) aftermarket "LGA775" hsfs don't blow air down onto the MB (but rather parallel to it), and blow in only one direction. If the Northbridge and PWM area have their own active cooling (fans), if there is high airflow over those areas from other fans, or if the CPU/MB is not run hard (that is, no gaming or other extended high-CPU use), this may not be a problem.
Bottom line: if you plan on CPU-intensive work like gaming, get a "flower"-style hsf, or be prepared to add on additional cooling for the Northbridge/PWM area that was designed to be cooled by the OEM hsf.