There is virtually no difference in heat, power, or noise characteristics of 5400 and 7200 RPM laptop drives. Even on battery the impact is essentially nil, & in a powered file server it's even less of a concern. You simply aren't going to notice any difference when it's in a case, so unless the cost difference is an issue just get the faster drives.
There are 2 or 3 good options in 320 GB 7200 RPM at around $90.
Viper GTS