By far the most cost effective upgrade is at least another 512mb ram for your PC. 256mb is just about enough to run Windows XP and to do anything else, particularly video editing, needs a lot more. The other essential is a good graphics card with it's own memory on board. Which particular type of NVidia card do you have?
Do you know that we're all in line for succession to the throne? Really? Well, if forty-eight million, two hundred thousand, seven hundred and one people died I'd be Queen.