Two EVGA GeForce GT 520 crossfire Vs. oneEVGA 02G-P4-2643-RX GeForce GT 640

Hey guys I have a GT 520 (http://www.amazon.com/EVGA-GeForce-Express-Graphics-01G-P3-1521-KR/dp/B004W3F9XO) and I looking to get a little more power. Would it be better to get anothr 520 and run it in crossfirer or buy this. http://www.newegg.com/Product/Product.aspx?Item=N82E16814130868&nm_mc=EMC-IGNEFL022113&cm_mmc=EMC-IGNEFL022113-_-EMC-022113-Index-_-DesktopGraphicsVideoCards-_-14130868-L0M

Im going to college so im on a buget but if yall have another card that works well fell free to show me.

 

One 640. SLI isn't supported by many games so it wouldn't be worth it.