The HD 7750 will be a good choice. If you want to add more info like budget and games you play I'll be happy to see if another card might be better
7750 is better then the performance of the 5770, 5770 = 6770, or gt 640 {may be better Then Them} only difference between the 7750 and the 5770/6770 is a very very minor performance difference and uses around 60w + requires no external pcie power connection. ]
7750 is the budget card dream for non power supply unit use. It's also useful for later down the road for upcoming games. Especially low budget gaming.
so my reccomendation is The Radeon HD 7750 is the best option since it is faster and consumes less power
R7750 >Gtx 650
http://gpuboss.com/gpus/MSI-Radeon-HD-7750-vs-GeForce-G...
see full spec here
http://www.msi.com/product/vga/R7750-PMD1GD5-OC.html
http://www.flipkart.com/sapphire-amd-ati-hd-7750-1-gb-gddr5-graphics-card/p/itmd7th7wdspnbrh?pid=GRCD7TGZBJSTR5YD&ref=f6335777-9c94-40ab-a308-1abbc2ff5577 Buy From here
or
http://www.amazon.com/MSI-OC-DisplayPort-PCI-Express-R7750-PMD1GD5/dp/B0077POOOY
or
http://www.junglee.com/MSI-Radeon-Express-Graphics-R7750-PMD2GD3/dp/B009SPJ8FG