Drive speed has very little impact on rendering time. Unless your project is DV only for source, cuts only edits, and you're rendering to DV (so that there actually isn't any rendering going on, just file copying), you probably wouldn't even be able to measure and difference in rendering time for different drives.

To help keep the temperature down, you could place an ordinary household fan next to the computer blowing on it. This will help make sure that cooler air is being drawn into the case by the computer's fans instead of recirculating the warm air that has already been exhausted by the computer. Placing it up on some small blocks so that there's more room underneath can help a lot too; the table underneath the computer makes a surprisingly good heat store.