the best bang for the buck is the gtx 750ti you will get like 100+ on total rome or dota 2 and is only 150 dollars. here is a link: http://www.amazon.com/EVGA-GeForce-Dual-Link-Graphics-02G-P4-3751-KR/dp/B00IDG3PRI/ref=sr_1_2?ie=UTF8&qid=1403210707&sr=8-2&keywords=gtx+750ti
i hope it helped you
^This. I'm barely using more than 4GB of system memory on games(per MSI Afterburner) so that upgrade won't do near as much for you as a 750 or 750ti would like joselo025 recommended. Also, then you have a solid card to add a better CPU/ddr3 ram to later on.