Very slow render time in FCP

I've got an early-mid 2008 Mac Pro. Just upgraded to 12gb of RAM, thinking that it would help my FCP render situation but unfortunately it didn't help at all. I'm here to ask what I should do next to speed up the process in FCP. I get a little lag during editing, but I have EXTREMELY slow render times. I usually edit on an external drive (seagate 2tb 7200rpm), but that alone still didn't help. (the internal 750 drives are even slower) Maybe my setup/pref is wrong, where I'm taking and saving my media? I'm not sure, and that's why I'm here. I'm curious what the next best step would be. Also, are the serial ATA internal drives still available (above 750gb)?

Thanks