The save error was from the 1.70 firmware... it's been fixed since the second day, just re-download and install
as for 2.56 vs 2.57 .. i dunno, the 2.57 has the dma boost option (some games don't like it)... but other than that they seem the same, and anyway it's not like you can't have both on at the same time.. I have the whole set.. (on install choose a different directory, I just add the number on the end)
though if you're having vaniaPOR trouble the remedy is to patch with 2.56 then 'back up' all roms to PC, format the mem card and then write all the roms back to mem card in one copy.. make sure to leave at least 64MB on the mem card free and also make sure vaniaPOR is not the last rom to copy (turn off auto arrange in windows, drag vania POR to top of list, edit/select all-> copy)