I was going to chk the Asus manual, there's usually specfic instructions for enabling Crossfire on a motherboard - but the site is down right now. If you reply to this thread tomorrow, I'll see it and try again.
These are the instructions for enabling Crossfire in my own Gigabyte MB, might apply to yours too.
1-6 Setup of the ATI Hybrid CrossFireX™ Configuration
Combining the onboard GPU with a discrete graphics card, ATI Hybrid CrossFireX can provide significantly advanced display performance for AMD platform. This section give instructions on configuring an ATI Hybrid CrossFireX system.
A. System Requirements
Windows Vista or Windows XP (Note 1) operating system
An ATI Hybrid CrossFireX-supported motherboard and correct driver
An ATI Hybrid CrossFireX-supported graphics card (Note 2)
B. Connecting the Graphics Cards
Observe the steps in "1-5 Installing an Expansion Card" and install an ATI Hybrid CrossFireX-supported graphics card on the PCI Express slot.
Plug the display cable into the onboard graphics port on the back panel.
C. BIOS Setup
Enter BIOS Setup to set the following items under the Advanced BIOS Features menu:
Set Internal Graphics Mode to UMA. (Note 3)
Set UMA Frame Buffer Size to 256MB or 512MB. (Note 3)
Set Surround View to Disabled.
Set Init Display First to OnChipVGA.
D. Configuring the Graphics Driver
After installing the motherboard driver in the operating system, go to the ATI Catalyst™ Control Center. Select CrossFire™ on the Graphics menu on the upper left corner and ensure the Enable CrossFire™ check box is selected.
(Note 1) For Windows XP, you must install AMD chipset driver version 8.51 or later.
(Note 2) You do not have to install the graphics card driver if the motherboard chipset driver has been installed.
(Note 3) To change the Internal Graphics Mode or UMA Frame Buffer Size setting in BIOS Setup, be sure to disable the CrossFire function in the operating system first.
I don't see two compatible cards for a crossfire configuration of any kind. The onboard video 3200 will only crossfire to a 3450 and a couple other similar cards. The 4550 will crossfire to another similar 4550 series card . There currently isn't any ati onboard video including 4200 series that will crossfire to a 4000 series dedicated card.