If you're gonna buy a high-end graphics card, make sure you got a decent power supply unit. since graphics cards use up power. Open up your cpu and look for the wattage (it should be located on the PSU). If the wattage is below 300 watts, better buy a PSU with a higher wattage. I recommend 350 watts or up. That should crank up the power supply to support all the parts of your system, including your new graphic card.
Refer to these sites If you need some more help:
http://reviews.cnet.com/4520-11319_7-6224085-1.html <--- also comes with video tutorial.
http://reviews.cnet.com/4520-3118_7-5023982-1.html