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#11
If you're curious, start performance monitor (perfmon), add the Available MBytes memory counter, and watch what happens to your system's RAM (consumed, not consumed, etc) when you edit video. If it drops down under 1GB, you need more RAM. If not, you should be fine. Yes, this is a very basic performance test, but it's good enough for "should I add more RAM or not?" type questions.
I will say that the more RAM you give Windows 7, the more efficient superfetch becomes - so if you can afford 8 or 16GB of RAM, regardless of whether you need it for video editing, it is a worthwhile investment.