I suggest get hd 7750.it is perfect for that resolution.hd 5830 is two gen old gpu with power hungry.hd 7750 is new and also it is good at direct x 11 games than hd 5830.you will able to play every game at high settings.
If you are gaming at 720p, then I agree with ASHISH65. Go for the HD 7750. It doesn't need a PCIe power cable*, uses less power, runs cooler, is more modern, and it is enough for 1366x768 at high(-ish) settings.
* I should say that most don't require a separate power cable. There are some on the market that do.