I use Imovie.....
all the time, for my own fun videos, and at school for my students, animation, and live action projects....it's an awesome and fun tool....but you def. need the memory....
That used to happen to me all the time, now the school gives me huge memory, and I also, have an external hard drive, so I can save all but the current project, on that.....but some tricks I've learned.....
Don't use any special affects, don't put any titles, don't add music......do your cutting, empty trash, and save and restart on occasion....and then, once the footage is where you want it...save it as a high quality quick time, it will not lose any quality, then you can dump the imovie file, (which will now be using a lot of memory, because of all the cuts) and restart a new file....bring in the Quicktime file that you made, this way, you'll have same footage, with less than 2/3 the memory used, and now, you can add music....but still, unless you have the big memory, and can save and delete all other movies that are on there, you have to keep it simple. Cuts, SFX, AND MUSIC ALL ADD TO THE FILE.