I'm looking to get a new PC for a job but would like to have something so that I can eventually do video capturing/processing/editing. My budget now is $250-300 so I realize that I can't jump straight into higher end components yet. Is it possible to build a base machine in my budget to get me through things now, and add a new component piece by piece?
(video card/Sound card/capture cards/extra HDs/etc.)

I've never built a PC before. I've had friends who built theirs and gave me some recommendations but I feel like I'm kinda jumping off the deep end a bit.

Should I simply buy a new desktop? If so, what would be the most important thing to get in a new machine so that I can do editing later?

Any suggestions would be very welcome.