Newegg.com is a good place to get good computer parts for cheap. They carry everything from an entire budget build under 200$, to graphics cards with a price tag more than your max budget. Shipping is usually fast, and their customer support and returns policy is usually very nice. The navigation of their site is very easy to familiarize with. Give them a try, you can get a very high end PC from them in around the 600$ range, not including the monitor/screen.