RE: Rendering
Every little thing can help of course.
As of now FCP can only use upto 4gigs of RAM no matter how much you have installed. But I noticed you only have 4 total. So heres a little breakdown of your existing RAM being used.
OSX - around 2gigs by itself
FCP - will use everything that is left if you have any other apps open.
So with only 4gigs with the OS and FCP alone there goes everything.
Now for a harddrive if you are using the same one that is installed with your iMac you are really working it hard.
Your harddrive is running an OS, what ever apps you have open, reading/writing video and then having to render all this in the end as well all at the same time.
So definitely get an external FireWire 800 HD. I personally prefer WesternDigital or GRaid. Lacie HD's can not handle hardcore use at all.
That way you are spreading some of the workload on the computer.
Also what ever external HD you choose make sure can set up as a Raid1.
WesternDigital has a 4TB MyStudio HD. And if you use the Raid1 format option you automatically mirror your files to both of the single 2TB HDs inside so if one crashes you simple buy another 2TB install it, then rebuild it to copy the info back the new HD and continue to work.
That bad part about this is you lose half your storage space but the good is if one crashes as your working everything is being automatically backed up on the fly.