Is it possible to run services/applications without the Gui?
E.g. to run a database Server like Firebird, without the overhead of a Gui to 'refresh' and therefor leaving resources as much as posible for the Database Engine?
Yes, it's possible. Just set the registry key HKLM\SOFTWARE\ReactOS\Windows NT\CurrentVersion\Winlogon\Shell to %SystemRoot%\system32\cmd.exe. (or cheat and copy file \ReactOS\system32\cmd.exe to \ReactOS\explorer.exe)