I wonder why you haven't done that already. Run out and get those additional sticks. If you are somewhat adventuresome, you can consider adding 3 x 4 GB sticks to achieve 18 GB. Just make sure that they are the same brand, model and timing specs.
What is your drive setup?
Read Harm on drive setup http://forums.adobe.com/thread/662972?tstart=0
- click the embedded picture in Harm's message to enlarge to reading size
- you need AT LEAST 2 drives for video editing, 3 is better
- some HD formats work better with (require) RAID
Now that's a little non-sequitur.
Why do you say that?
Memory is only PART of the editing experience
Number and configuration of hard drives is also important
The hard drive setup is unlikely to cause the described 'artifacting' . The question was about the benefits of adding memory. Asking about hard drive seems unrelated.