If you want to make use of two drives, use one for for removeable backup.
The only thing it could help in games, given sufficient RAM and adequate CPU is load times and what and that is not money well spent compared to on actual game-play performance from a better viddy card and such. What difference can it make anway -that between 20s and 10s in the best case? A higher performance single HDD (higher densitiy, higher RPM, etc) could negate any difference compared to two lesser HDD's and without any of potential problems.