You need both extensions, IOATAFamily gives generic ATA protocol support (plus support for specific controllers like Intel ICH) and JMicronATA gives support for specific JMicron controller.
A kext can support only 32 bits, only 64 bits, or both 32 and 64 bits. Older Leo kexts are mostly 32 bits only, newer patched kexts like the patched JMicron could be 64 bits only. Most Apple's vanilla kexts support both 32 & 64 bits, like
IOATAFamily is 32/64, but there are reports it causes kernel panics on certain ICH chipsets when working in 64 bits. There is a patched IOATAFamily thats supposedly corrects that. Either the original one or the patched one should work in both 32 and 64 bits.

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AsRock P45TS | C2D E8200 | GeForce 8600GTS
Mac OS X Snow Leopard 10.6.1 + Windows 7 Ultimate 6.1.7600 + Fedora 11