Found this with a little google (didn't test it though):

Code:
uses psAPI;

procedure TForm1.Button1Click(Sender: TObject);
var
  pmc: PPROCESS_MEMORY_COUNTERS;
  cb: Integer;
begin
  cb := SizeOf(_PROCESS_MEMORY_COUNTERS);
  GetMem(pmc, cb);
  pmc^.cb := cb;
  if GetProcessMemoryInfo(GetCurrentProcess(), pmc, cb) then
    Label1.Caption := IntToStr(pmc^.WorkingSetSize) + ' Bytes'
  else
    Label1.Caption := 'Unable to retrieve memory usage structure';

  FreeMem(pmc);
end;