Can anybody tell me the basics when buying a video/graphics Card?

Answer 1

Part of this article might help:
http://www.directron.com/howtovideo.html

Take note that gaming performance is a lot dependent on:
1. Graphics processor architecture - More pixel pipelines or stream processors, vertex shaders, etc. the better. Most cards support DirectX 9.0. New cards now support DX10 which is only in Vista.
2. Higher core clock, the better.
3. Higher memory clock, the better. 256Mb ddr3 will always beat 512Mb ddr2.

If you are now on Vista or plans to upgrade to Vista shortly, consider this new DX10 card that has performance reserve that you can unleash:
http://www.vr-zone.com/?i=4925

Answer 2

well i donno bout basic but this info will really help....
1. make sure you got your agp and pci express right
2. see if the graphics card needs a more power than the slot can supply, youll see a connector on the back or top of the card.
3. make sure that it will suply what you want to do with it. ie.) dvi connections, SLI, crossfire, DX10, shaders, ECT.
4. make sure it wont burn a whole in your pocket.
5. look at some of the special features on the card.
well thats all i can think of right now.

TIPS:
ATI makes good AGP cards while Nvidia makes good PCI express