for aorund 650$ you can get a pretty nice setup, and seeing you alrdy have a gfx card makes it that much better. here is a pretty nice build for 536$ with no case, (dont know what you like in a case so i left it to you to pick one) and also befor mail in rebates.
mobo
cpu
ram
psu
HDD
and heres link to case for your pick, and with the price of only 536$ you can get a really nice case or get a lower end one and get another HDD or w/e you want to upgrade.
cases