xen-
Pentium 4 @ 3ghz, 2gigs of ram.
Video is smooth, but you see flaws when running full-screen viz in Media Center. Visualizations look pixelated and chunky. Hairstyle has similar issues - dunno if this is because of the aspect ratio or resolution or some combination of both. High res will bring out the flaws in anything.
If you have Windows set to show window contents while dragging, it is not perfectly smooth. But it's not wretched by any means; just a little 'shimmer' while moving windows and a brief but noticable refresh delay for parts of the screen that get revealed as a result of moving a window. But I turn all UI effects and animation stuff off (me = impatient b*st*rd), so I never see that.
DVD playback is... just... sweet. Obviously there's some letterbox because of the difference in aspect, but image quality is... sweet. Usually I don't switch resolution when watching movies full-screen. I just let it interpolate. The rare 'stutters' are due to poor formatting/layout of the disc or layer-switch, and are usually visible with stand-alone players, ie. not the Radeon's fault.
Games are smooth. The monitor has no problems with resolution switches, even from 4:3 to widescreen and back.
Though dual monitors are attractive - it would be nice to separate interface stuff from content being worked on - I have seen too many friends with "Why won't it show up where I tell it to?!" issues. Widescreen gives you a place to put menus, toolbars, etc. and still be able to see what you're working on. It's extra nice for any apps with a timeline - audio editors, Flash, video editors, etc.
Keep in mind that this is a 9500 driving the monitor - YMMV and possibly be better with a newer, higher-level card. Sorry for making such a tome out of this post
EDIT: just to make this post a little longer...
Video cards that I found unacceptable for this system:
3DLabs Oxygen VX1 Pro - screen refresh too slow.
NVidia GeForce 4 (don't remember exact model) - unacceptable image quality.
Matrox Parhelia - buggy, unstable piece of...
ATI Radeon 8500 - unacceptable image quality.