You did, but only a brief description. We have no idea how fast your RAM is running at, what model the motherboard is to chose a CPU you can upgrade to, Hard Drive and a few other things.
But here's the facts -
Your Motherboard is a Socket 939 by AMD - AMD does not manufacture Socket 939 CPU's anymore so you can't upgrade you processor.
Your RAM is probably at a low speed so you could spend about 20 buck's and upgrade to faster RAM (If the RAM isn't the fastest DDR speeds can go. That computer uses DDR RAM and not DDR2 or DDR3 so the fastest you can get is DDR 400Mhz RAM)
You probably have a integrated graphics processor(Integrated Into the motherboard) so that is why your gaming graphic's are not that great.
The motherboard of a old model so I would think it would have a AGP Graphic's card slot inside. AGP is a outdated slot and not many cards are made for it, but for CSS and HL, that's really all you need as those games are highly optimized for old systems etc. As for your video editing, if you edit very large files etc, you would need to upgrade your motherboard to one with a PCI-Express Graphic's slot. To do this you would have to get new RAM and a New CPU. Not sure about a new Operating System though.
Overall, if you want to upgrade anything in that Computer besides the video card, you would have to pretty much build yourself a new one.