More information about headers and why I think OBX headers is the equivalent of throwing your money in a fire
Article1
Article Main page
Basically, stay away from formed tube collectors, unequal length tubes, and check the R&D done. As for R&D to look for, check the list below
You want a merged head collector that is at least 5" in length so the transition is smooth.
You want the primary diameters sized correctly to the engine output
You want primary tube lengths sized so they are tuned to the power band of the engine (pressure wave tuning as well),
Stepped headers can also increase power and improve the usable powerband, but the steps must be placed correctly, not just added for the sake of being stepped.
Check craftsmanship, welds, materials used, see if anyone had ill fitting headers, etc
Get them ceramic coated, it lowers under hood temps and improves performance.
DON'T GO CHEAP!