You don't really need any fancy graphics card for your usage. But if dealing with HD content, then cards that support related software features can help to speed things up.
I believe the technologies are called CUDA click here;filter=Video%20&%20Audio & PureVideo click here for Nvidia-based cards. And Stream click here & Avivo click here for ATI-based ones.
Of course, a fast processor & good amount of RAM are the main ingredients. So I would try to get the fastest CPU & most RAM (up to a limit) I could afford, rather than being too concerned about the GPU. I wouldn't spend more than £50 for this - would consider something like a NVidia 9500GT / ATI HD4650. Or even the newest ATI HD5450 for a similar price would do. G