The amount of RAM is one of the biggest deals.
I assume you actually mean "editing" and not simply "converting" as converting is CPU-limited.
*If it's really critical, an SSD may be the better way to go. Also, some laptops support TWO drive slots allowing a smaller SSD to be used with your video software (you need to have it setup properly as a scratchpad and the video stored on it or whatever is recommended.)
**it's difficult to give good advice without knowing more about what you're doing. Simply telling you to swap your 5400RPM for a 7200RPM drive might not be the best advice. The easiest advice is to tell you to MAX out the RAM that your laptop and Windows version supports (16GB is the Windows 7/8 Premium limit though many laptops may have a 8GB physical limit of 2x4GB.)
I can't thank you enough for your wise advice; I'll follow it! (ALL of it!)