[PhoeniX]

Код:

var
  Stream: TStream;
begin
  Stream:=TFileStream.Create('c:\Downloads\chupakabra.txt', fmOpenWrite or fmShareExclusive);
  (Stream as TStringStream).WriteString(UTF8Encode(Memo1.Text));
  Stream.Free;
end;

ок, не? Если не, пробуем ANSItoUTF8 вместо UTF8Encode...