Hello Ibreakthings,
WD Greens are IntelliPower drives, which is fancy way to say they are spinning at 5400 rpm. I'm not very familiar with the Fraps codec particularly but (please correct me if i'm wrong) it's close to uncompressed format. Lesser video compression requires more HDD performance. So no, Greens are not fast enough. I would recommend a 7200 rpm drive for better performance, like Seagate Barracuda or WD Blue. The price should be somewhat the same.
Quick edit: Actually Green should be enough for you because you because you are not going to record 1080p, but... you know... IntelliPower... There are better choices.