That shouldn't be a problem actually. Apparently, FF III is 64 MB compressed, FF IV 83 MB compressed, while Kingdom Hearts 358/2 is about 200~ish MB compressed. I'm quite confident that possible FF V and FF VI remakes for the DS wouldn't be a problem graphics-wise. I don't know exactly how much capacity a DS cartridge has, but after googling some info it seems to be in the region of 512 MB to 1 GB. If anyone can find an absolute 100% answer to this, some information and a source would be good, because I'm actually pretty curious about it myself. In terms of processing, Kingdom Hearts 358/2 runs just fine and graphically looks quite good considering, so I don't think a possible FF VI remake has anything to worry about.