Newer 7200 RPM drives don't really generate much more heat than a 5400 RPM one. Gains are variable depending on the game. I have an SSD in mine and saw a ~44% reduction in load times with Gran Turismo 5 from launching the game to entering a race.
Another option is one of the new
Seagate SSHD drives (traditional drive+8 GB cache SSD). If you play one game at a time, it would have more of an effect.