Online games are generally not very graphics heavy and don't need a good graphics card. Your crashes are probably due to something else. I have the exact same computer right now so I could try out the games you are having trouble with if you tell us what they are.
If you still would like to upgrade, the Radeon 6670 GDDR5 low profile cards are the best you can put in your system and it costs about $100. But there are cheaper ones as well.http://www.newegg.com/Product/Product.aspx?Item=N82E16814102958