I'm getting around 440MH/s out of mine. I can't remember where I saw it, but there was a chart comparing MH/s, cost, and power consumption and the 5870 is THE card to get for MH/s vs. Power used.
If you add up the costs for a server (bare minimum)
cpu- $30
ram- $10
psu- $150
mobo- $140 (if you know cheeper then this with 4 pci express then let me know)
hd- $15
PCIe x16 Extender Cable $12*4
5870 $190x4
= $1,153
(440x4)/1153=1.53 megahash per dollar(full server cost)
with 5830
5830 $110x4
= $833
(330x4)/833=1.58
So as long as you go the absolute cheapest route 5830 rig at 110 is better but not much so. If you add a case or more to the cost at all the 5870 at 190 will probably overcome the 5830.