Well I am getting 315Mhash/s on my Radeon 5830.
Using Traxx Overclocking Util. Mem on 300 GPU on 1022
Phoenix 1.50 AGRESSION=13 W256 V
People aren't understanding the topic, so I'll use you as an example. Your 5830 gets 315Mhash/s, but it sucks about 230w of power. A 5770 gets around 180Mhash/s, but only sucks about 120w of power. So let's set up a simple proportion here.
5830=
315Mhash/s 230w
Total- 1.370Mhash for every 1 watt
5770=
180Mhash/s 120w
Total- 1.5Mhash for every 1 watt
In summary, with a 5770, obviously you don't get bitcoins as quickly, but you save enough power to make up the difference and more.