This really depends on your monitor too , but I think you are going to be OK on at least medium settings by installing a 70-$80 graphics card .
I cant see full details of your case , but I believe it has a free graphics card slot , and will accept half height cards . The power supply is fairly limited too but you should be alright with
MSI R6570-MD1GD3/LP Radeon HD 6570 1GB http://www.newegg.com/Product/Product.aspx?Item=N82E16814127585
HIS H657H1G Radeon HD 6570 1GBhttp://www.newegg.com/Product/Product.aspx?Item=N82E16814161374
The MSI is the better option because it has fans to cool it , but you need to check there is room for it in your case
should give you a fair idea of how well a card like this performs . 30 fps is good enough for game play and any thing above 50 is silky smooth . You can adjust quality settings to give you good visuals and good enough frame rates
Both cards include an alternate back plate to let them fit in slim cases .