These work:
http://www.addonics.com/products/adebide2sa.phpThere may be others. These are the ones I tested - and used to replace our Promise 4 port sata cards.
Know we're now on the edge of financial viability here....If you were starting from zero, might be more cost effective to find an atom board with gigabit and 4ch sata onboard. But I guess it depends upon what junk you have lying around - because those atoms aren't cheap. They should be - but they're not.
How far will these adapters take you? (shrugging) Only tested to 16TB this iteration. I'm using the Hitachi/WD 7K4000 drives. Your mileage my vary with others.
Flogging/filling now. Pretty sure that PCI bus is saturated. Not elegant. Works.
FYI: I'm still interested in an atom board upgrade for these things. Needs to be <$100 fully populated. $60 would be better. Next tip is on you