I'm sure it's different for Europe (they got FFIV/V/VI in one package?), but in NA:

Origins: I & II (Enhanced WSC graphics and I think orchestrated music. Or it's really well synthesized. One Disc, with bonus artwork/bestiary/collections you can work for.)
Chronicles: Chrono Trigger and IV (Normal Graphics. They have horrific saving times though...bonuses on the CT Disc.)
Anthology: V & VI (Normal Graphics. Different sound chip somewhat downgrades music quality. Bonus stuff on the FFVI Disc.)

FFIII will be remade on the Nintendo DS and released later this year.