One 5830 will actually pay itself off in 2 weeks.
The 5830 averages 286 Mh/s in a survey of 20 of the entries on the Hardware Mining Comparison Chart.
The 5850 averages 236 Mh/s in a survey of about the same number of entries (can't remember now). That is an incredible difference!
I guess it can be attributed to the fact that the GPU used in the 5830 is the same one used in the 5870.
Right now Amazon has a 5830 for $154 and a 5850 for $161. Very small diff in price, but big diff in Mh/s.
you'll want to start by using actual comparison numbers.
a 5850 at 236 Mh/sec? ummm... no. i don't actually believe they'll go that slow.