You may be sacrificing some capacity for real HDD speed. When you start to consider seek times and such... platter size comes into the equation at some point. My WD 640GB drive was a fairly fast HDD at the time of purchase 5 years ago because it only had 2 platters of 320GB in size. This probably still applies to your search.
I also had a couple WD Velociraptor 74GB hard drives back in the day. I never bought into the 150GB or 300GB versions though. The speeds they hit aren't that noticeable in gaming anyway to be honest, compared to a typical 7200RPM HDD.