이런 방법도.....

void __fastcall TForm1::Button1Click(TObject *Sender)
{

static int nI = 1;

Memo1->Lines->BeginUpdate(); // 또는 ::SendMessage( Memo1->Handle, WM_SETREDRAW, 0, 0 );
Memo1->Lines->Add( nI++ );
Memo1->Lines->EndUpdate();   // 또는 ::SendMessage( Memo1->Handle, WM_SETREDRAW, 1, 0 );
}