Upgrading my personal Computer???

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I am a interested in digital video editing and my computer is a six months old pentium 4 ,with 256 mb RAM ,Asus mother board 80 GB hard disk.I am not able to work in Avid D.V express pro with this configuration and am planning to upgrade it.But I am confused there with the wide range of products available in the market.I am planning to buy a more capable RAM and a more storing hard disk.Also an external hard disk and a fire wire port.May be I will be needed to change my motherboard to a Dual core processor.Which is the best brand in market if I want to buy a video capture card in a normal range I mean in a price range of about $200 - $250.And please tell me what brand and in how much power I have to buy those things.