If I'm doing my math correctly using this online calculator:

http://www.alloscomp.com/bitcoin/calculator.php

Difficulty is at 877266, and a 5830 will yield approx 240Mhash/sec without overclocking, maybe 265Mhash/sec for an average overclock.

If you purchase a new 5830 for approx $130, it will not pay for itself in a month.  This online calculator doesn't include difficulty increases so it's sort of a "best case" scenario.  Next difficulty increase will likely be over 1,000,000 and will likely continue to be 50% to 80% increase every week or 2, so by the 2nd month that 5830 would make about $70, 3rd month, about $40.  Then we take into account the rest of the mining rig, electricity, air conditioning, shipping costs, etc.

I guess for now I'll keep running my existing rigs, but not plan any expansion.

Does this seem right?  Will we see a flood of used ATI cards on eBay?