Both
ATi and
nVidia have done great things recently regarding video quality, so if you get a discrete video card with either of these companies chips you will be fine.
That said, one of the single biggest issues over the past couple of months is about HD-TV, and in particular HDMI connectors.
I have yet to see a readily available graphics with a HDMI connector, pretty necessary for "HD-Ready" televisions and Blu-Ray or HD-DVD based video (well actually latest news is that Blu-Ray may not require, but that's a different story).
You may therefore be best advised not to buy any graphics card right now, as the situation will almost certainly change over the next month or two.
A DVI connector can of course display high res (and will be fine with DVD as a source), but just not HD-DVD because of the encryption side of things.
Beyond that, pick a bottom of the range (ie cheapest) card from either of these two companies, unless playing current 3d games is a priority.
Make sure that it doesnt have a fan on it (which generates extra unwanted sound).
I'd personally recommend you spend around £40 to £50, no more.
Currently, a passive GeForce 7300 costs
£43, or
£45 for a similar ATi card. Be aware neither of these have HDMI connectors, but do have "S-video/TV-out" for standard resolution TV's.