mx4000 on asus Motherboard?

Your mobo has an AGPx4 slot and will accomodate that new card without any problems.
That card is AGPx8/x4/x2 compatible, so no problems.
Check if there is a jumper on the mobo, or a setting in the BIOS, to switch off the onboard graphics. Some mobos switch it off automatically when you insert an AGP card.
Before you put the card in, uninstall the current graphics drivers and load the most basic, windows-own VGA drivers. Then switch off and do the swap.

 

If it was me, I'd spend the extra $10 and get the one right below it. Faster memory. more memory bandwidth and more pipes. 256MB DDR vs. 128MB memory. Also it has vertex and pixel shaders, the 4000 has none. Plus about 16 million more transistors. Definitely worth the extra little bit.

http://www.directron.com/sa5200256.html