One site that will help a lot is http://www.newegg.com/
They have good deals on all items.
For the video card I would suggest an nVidea card preferably in the 8000 or 9000 series. The CPU is not as important as the rest of the PC but still get as good as you can with the money you have. I only have an ancient 1.8 GHz single core Pentium IV Processor and I can run Crysis well with some things set to medium and high with relatively no lag.







