http://www.promise.com/product/produ...=97&familyId=3
This isn't a RAID card - it's S-ATA and ATA.
The point I'm making is that with this controller, you can use two ATA HD and two S-ATA HDs at the same time.
So you could use your existing ATA HD (I'm assuming your existing HD is ATA) and then decide whether your new HD is ATA or S-ATA.
Either way you're covered. You have backward compatiblity for the old HD and have future-proofed yourself, in case you wish to use S-ATA, now or in the future (well the foreseeable future, anyway).
ATA and S-ATA use different cables - but this card will cope with both types.
I don't know what you mean by "which is mainstream"?