How can I check system specs in Inno Setup before/during installation?

In particular, I'd like to check the amount of RAM installed on the PC. If it's less than 1GB, I'd like to show a warning message before/during the installation...

I would personally do this check at the beginning of the setup to not disappoint the user going through the installation wizard. To do this, I would display the warning before the wizard is actually shown as its used in the following script. It should warn the user when the memory is below 1073,741.824 B (1GB) of physical memory detected on the machine. If the user disagree with the warning, the setup is terminated; if there's more than the mentioned amount of actual physical memory or the user accepted the warning, the setup process continues:

[Code]
type
  // the following mapping of the DWORDLONG data type is wrong;
  // the correct type is a 64-bit unsigned integer which is not
  // available in Inno Setup Pascal Script at this time, so max.
  // values of the following fields will be limited to quite a
  // big reserve of 8589,934.592 GB of RAM; I hope enough for
  // the next versions of Windows :-)
  DWORDLONG = Int64;
  TMemoryStatusEx = record
    dwLength: DWORD;
    dwMemoryLoad: DWORD;
    ullTotalPhys: DWORDLONG;
    ullAvailPhys: DWORDLONG;
    ullTotalPageFile: DWORDLONG;
    ullAvailPageFile: DWORDLONG;
    ullTotalVirtual: DWORDLONG;
    ullAvailVirtual: DWORDLONG;
    ullAvailExtendedVirtual: DWORDLONG;
  end;

function GlobalMemoryStatusEx(var lpBuffer: TMemoryStatusEx): BOOL;
  external 'GlobalMemoryStatusEx@kernel32.dll stdcall';

function InitializeSetup: Boolean;
var
  MemoryStatus: TMemoryStatusEx;
begin
  // allow the installation (even if the GlobalMemoryStatusEx call fails)
  Result := True;
  // that's the requirement of the function call; you must set the size
  // of the passed structure in bytes
  MemoryStatus.dwLength := SizeOf(MemoryStatus);
  // if the GlobalMemoryStatusEx function call succeed, then...
  if GlobalMemoryStatusEx(MemoryStatus) then
  begin
    MsgBox(Int64ToStr(MemoryStatus.ullTotalPhys), mbInformation, MB_OK);

    // if the amount of actual physical memory in bytes is less than
    // 1073,741.824 B (1 GB), then show a warning message and according
    // to user's decision abort the installation
    if MemoryStatus.ullTotalPhys < 1073741824 then
    begin
      if MsgBox('You have less than 1GB of physical memory available. ' +
        'Are you sure you want to continue with the installation ?', 
        mbConfirmation, MB_YESNO) = IDNO
      then
        Result := False;
    end;
  end;
end;