Since you have Windows 7 Ultimate with Windows XP Mode installed, you already have virtualization software: Windows Virtual PC (2010).
You can use this virtualization software or you can install and use other virtualzation software like VMware Player, VMware Server, or other.
You will however need Windows 2003 Server media (CD/DVD) and a license in order to run a virtual Windows 2003 machine. Unlike for Windows XP, you don't get a "free" license for Windows 2003 with your Windows 7 Ultimate license.
Of course I'm not going to use both at the same time.
You can run more than one virtual machine at a time, provided you have CPU power and especially memory (RAM) enough.
EDIT: You will not get the same level of integration with your Windows 7 desktop like you have with Windows XP Mode - i.e. you will be able to run a Windows 2003 Server in a window containing the Windows 2003 desktop, but you won't be able to run applications installed in your Windows 2003 Virtual PC as if they were Windows 7 applications by starting them directly from the Windows 7 Start Menu (like you can with the Windows XP Mode applications).