Most HSF's are setup to blow. The design of the heatsink has a lot to do with which way cools better (thus, some alphas work better with fans setup to suck air, while other heatsinks work better with fans set to blow). I believe the APC was designed for a fan to blow down onto the processor (the design of the radial fins curving around seems quite "aerodynamic"). However, you should setup MBM 5 (or some other monitoring program) and try it both ways. Run it through prime95 for a few hours, and compare.