A600 and A1200 uses 2.5" IDE drives, hence the smaller connector. The connector provides the usual 40 pins plus 4 extra, mostly power.
You don't need anything except a drive and a cable. If you go for 2.5", use the cable you have. 3.5" drives needs to get power from somewhere and you need to convert the size of the connectors with some form of special cable. Or a regular kit mounted backwards. I use this, but it won't fit unless you have towerized the machine.
A stock A600 or A1200 won't go for a very large drive. The filesystem can't handle partitions larger than 2G, and the device driver can't handle disks above 4G. But with a software upgrade to OS3.9 this becomes so large I don't think you'll have a problem any day soon. We're talking huge, ok? ;-)
Good luck
