If the GTX 660 is sold out or not available at that price, I suggest either the HD 7850 2GB for about $170 after rebate or HD 7870 for $195 after rebate or another GTX 660 also for $195 after rebate. You will still be within your budget.
The total should add up to be $524 after promo codes and rebates with that $180 GTX 660.
Edit: If you don't mind buying from Amazon you will be able to save another $20 on the same case and this pair of RAM for the final price of $504. If you're willing to go over your budget by $4, you can get a quad core i5 for $180 from Amazon for a great gaming rig for the price for $579 after all the promo codes and rebates.