|As you have not told us anything about your system it is difficult to advise you correctly.|
Video cards come in different flavours
1. on-board video - This is integral to the motherboard. Some have BIOS configurable memory settings which can be increased at the expense of your RAM, They can be disabled in favour of an add on card.
2. PCI - These are old style graphics cards used in PCI slots before the advent of AGP slots.
3. AGP - Motherboard has an AGP slot into which it is slotted. There are different formats and speeds of AGP cards. For example AGPx1, AGPx2, APGx4 and AGPx8 which refers to its speed interface. They also come in different voltages with the earlier cards being 3V and later ones being 1.5Volts. Not all AGP cards are interchangeable
4. PCI-E - PCI express - These are newer format and slot into a newer style PCI-E slot on the motherboard.
If your system is a laptop then it is another story altogether.
Here is some sample how tos:
Also read this for future reference as well:
☺ When everything else fails, read the instructions.