I think just about any AGPx2 or AGPx4 card will do for just video display. The ram won't matter much since anything above 32meg is only used for 3D rendering.
For video editing you need at least 16 bit color with 24 bit prefered. Display resolution should be at least 800x600 with 1024x780 usual. More is even better if you have a large monitor.
A dual monitor capable card comes in handy, letting you have your editing program in one and your actual video showing in the other.
I use an ATI Radeon 9000 Pro with 128meg ram. It has dual monitor and NTSC output which comes in handy from time to time.
The core frequency doesn't have much bearing on video work, only on 3D games does it count.
Today's video cards are almost seperate computer systems, ones that are highly optimized for making 3D graphics. For video editing it's a waste, since all the actual editing is done by the main computer and only the results are displayed with the video card.
Fav quote - "Experience is whatcha don't get 'till ya don't need it no more."
System - Athlon 1.4GHz, Win98, Hauppauge PVR250 receiver and compressor.
Software -Magix Movie Edit Pro 10, Nero 6 + NeroVision Express, Moho 4.61, PSP 8.1, Bryce, Quicktime 6.52 pro, Goldwave 5, DVD-Lab.
Cameras - Panasonic GS9, Canon ES8400V, Canon EOS D20 and Canon A70