The problems are HDD crashes and power supply failures, point taken on config of current dead box which is a Quad core CPU, bearing in mind your comment the iMac would perform no worse..
If I went for more RAM 16MB seems to be the limit I sense I should be in a similar performance bracket. Having done some video editing looking at the Mac software will this be as good as the current (high end) offerings that are available and by this I do not mean MS Movie Maker and similar software..