Graphics card provides display output to your monitor. There are many graphics card manufactures out there who release a new card into the market nearly every three to six months as it is one of the most competitive hardware market. While choosing a graphics card consider what you would be using it for. For 2D applications such as word-processing, spreadsheet etc, a basic graphics card with around 256MB or 512MB video memory will be sufficient for your needs. It will also allow you to run your monitor at various resolutions.
However, if you require 3D functionality for running 3D applications or playing 3D games you will require a 3D graphics card with at least 1GB or even 2GB+ of video memory.
You can buy graphics card with a PCI , AGP or PCI-Express bus. However, most current motherboards have PCI-Express and few have AGP slot. You should therefore use an AGP or PCI-E graphics card if your motherboard has an AGP or PCI-E slot.
PCI Express graphics card offers much higher graphics bandwidth which results in higher performance. You are recommended to buy a motherboard with a PCI-Express port so that you can use a PCI-Express graphics card. The current top end graphics card are made by Nvidia (Geforce cards) and ATI (Radeon cards).
Recommended Graphics Cards:
MSI NVIDIA GeForce GT 610 1GB GDDR3 VGA/DVI/HDMI Low Profile PCI-Express
GIGABYTE GeForce GTX 660 Ti WINDFORCE 2X OC Edition 2048MB GDDR5
EVGA GeForce GTX 660Ti SUPERCLOCKED+ 3072MB GDDR5 DVI-I, DVI-D, HDMI, DP
Asus ATI Radeon HD6450 Silence 1 GB DDR3 VGA/DVI/HDMI Low Profile PCI-Express
Gigabyte AMD Radeon HD 7850 2 GB GDDR5 DVI-I/HDMI/2x mini-Displayport PCI-Express
Sapphire Radeon HD 7950 3GB DDR5 HDMI / DVI-I / Dual Mini DP with Boost PCI-Express