First, here is a thorough guide on installing a graphic card. http://youtu.be/O9x097QRXeA
And, to choose a graphic card, we need to know your system spec. The crucial components is CPU, Motherboard and Power Supply.
This is the most powerful graphic card for $65. SAPPHIRE Radeon HD 6670 1GB 128-bit DDR3 PCI Express 2.1 x16 HDCP Ready Video Card