Ok, your getting more and more advanced, and I'm not very advanced, but I'll see what I can help with.
I got 2gigs of ram and film onto my own harddrive, it lags sometimes, but usually not. I suppose u should (if u can afford it!) upgrade to 2gig ram. It will also come in handy when playing games and doing stuff like that.
The higher u put the fps (frames per second) the more it will take of ur ram. I use 30fps and it works pretty fine. I also render the movies with the same fps, so it's fine for me. If it lags u can however put it lower and see if that works better. See Fony's filming blah, blah, blah guide (
this one). I think he describes it better.
And keep on asking if u need help, I'm no real pro, but I will do my best to help u.