First of all, go to Memory Express (
www.memoryexpress.com) to buy your parts. Great prices, and the best salespeople in the city.
If you are comfortable with it, buy an AMD processor. Great performance and cheaper than Intel. Make sure you get a dual core processor for both the gaming and video editing. The second most important thing for your requirements will be the video card. This is also going to be the most expensive part you buy, so do some research. I am an Nvidia fan personally, but get the best that you can afford. I would suggest something with 512mb of ram for gaming.
ONe thing that a lot of people overlook is the power supply. Computers these days are power hogs (especially video cards) so get something decent. 400 Watts if you can afford it.