HD speed doesn't determine rendering...
Your hard drive just stores and plays back your footage. "Rendering" time is determined more so by processor speed and your graphics card's vram.
Normally 7200 RPM drives are better at writing and reading footage than 5400 rpm drives. There are some instances where 5400 RPM drives have outperformed 7200 RPM HDs, but that's when the 7200 RPM drive is more than half full and other similar conditions.
It's better to use a 7200 RPM Firewire (NOT USB) drive as a scratch disk. If you're using ProRes or DVCProHD codecs you could probably get away with a single drive, FW800. But if you're using 10-bit uncompressed HD or something like that you'll need a RAID, which is two or more hard drives in a case, single enclosure or MacPro's internal bays that work together to spread the footage over the multiple drives for increased read/write speeds. There are a few different RAID configurations, RAID 0 is probably the easiest, most common one for increased performance.
The more layers you're compositing with filters and effects that you're trying to play back in realtime out of Final Cut Pro can tax even the most robust systems.